WASHINGTON WAGS with an eye on past scandals have taken to re-phrasing the central question of Watergate to fit the Iran-contra affair: The question, they say, isn't "What did the president know and when did he know it?" but "What did President Reagan forget and when did he forget it?"

At 75, Ronald Reagan is our oldest president. Periodically, questions about his age crop up. In the 1984 campaign, when he appeared to founder during the first debate with his Democratic challenger, questions were raised about his grasp of facts. Questions are being raised again. To put the matter more bluntly: Is the president suffering from senility? Can his judgment be trusted?

No doctor, no matter how skilled, could reasonably make such determinations without the opportunity to personally examine the president at length. But one important point should be made: the president's mental fitness can't be determined on the basis of an evaluation, however thorough, of his powers of memory. The question of aging and fitness for leadership is far more complicated than that.

Let me offer a simple judgment: Based on what we know, there is no reason to believe that Ronald Reagan isn't up to the job. His obvious memory lapses don't provide the basis for a broader judgment about his mental fitness.

If the topic is senility, memory is only one part -- and a small one at that -- of such an evaluation. Senility is the layman's term for what physicians refer to as dementia, which describes a specific set of impairments. Besides memory, particular attention is given to orientation, language, fund of knowledge, capacity for abstract thought and judgment.

Typically, a person suffering from early dementia shows the following characteristics:

They lose interest and enthusiasm.

They are frequently distracted.

Their communications lack clarity.

They suffer from a loss of suppleness and spontaneity.

Their vocabulary is restricted.

They are subject to radical mood shifts.

Their judgment is impaired in regard to appropriate courses of actions.

It is also important to remember that dementia -- senility -- is an illness, the result of disease, and is not a consequence of normal, healthy aging. Any one of us could exhibit one or more of these symptoms. When over-worked, over-tired or over-stressed, it's easy to lose the thread of our conversation or to become upset over something that ordinarily would be a minor incident. Dementia is characterized by the intensity and pervasiveness of these dysfunctions.

Our question about the president is not likely to be answered, therefore, by simply ticking off examples of those occasions when he may have been less precise and specific than we might have expected. It might be helpful, instead, to focus on the problem that is most discussed whenever the issue of senility comes up: memory.

Americans are fascinated by memory virtuosos. Best-selling books have been written on the subject of how to develop a super-memory. We don't expect any less in our presidents. John F. Kennedy and Jimmy Carter -- to mention only two -- exhibited super-power memories. At press conferences they were able to dredge up from their memories facts and figures that in retrospect didn't always seem that important. But at the time their memory virtuosity held us spellbound.

President Johnson, in contrast, was disappointing when it came to the exercise of brute memory. President Reagan also favors vague generalizations rather than the recitation of crisp, precise information.

Although some might make jokes about it, memory for most people is no laughing matter. Everyone experiences forgetfulness from time to time. And everyone has his or her pet theory to explain their memory lapses. "Young people forget and say they are busy; older people forget and get woried," as one psychologist put it.

But memory is also a worry to those in the middle years. According to a recent national poll by the University of Southern California, 44 percent of Americans over age 45 fear that they may eventually fall victim to the most dreaded forgetfulness-associated illness in history, Alzheimer's Disease.

Despite the growing interest and concern about the disease, research shows that probably no more than 3 to 6 percent of the population between the ages 65 and 79 will get a dementia-causing disease; for those 80 and above, the rate rises dramatically to 20 percent.

But we all know that older persons don't seem to remember things as well as younger people. Or do they? Most studies in the elderly show that the majority of people in their 70s -- although by no means all -- develop some degree of memory impairment as they age. The impairment however, is highly selective.

For instance, the ability to acquire and store new information -- learning how to operate a new appliance or learning the way around a new apartment complex -- proceeds at a slower pace. Once learned, however, there is no indication that an older person's grasp or ability is any less than his or her younger counterpart's.

Most impaired in the elderly person is what psychologists refer to as "episodic short-term memory." This capacity is tested by such tasks as freely recalling long lists of numbers or words. An older person often does poorly on such tasks, particularly if the list is in the range of 25 words or more. He or she performs even worse if the words are totally unrelated and can't be separated into categories.

Why is short-term memory impaired in older people? Memory experts believe that a major factor may be the longer processing time that older people require in order to retrieve something from their memory. Older subjects also take longer to register or encode non-verbal material such as maps or abstract designs. There are also differences in the way they go about memorizing.

An older person, on the average, is more likely to have been taught only rote-memory methods in school. Younger people, in contrast, are more likely to have come into contact with mnemonic systems: the use of memory tricks such as the clustering of words according to similarities in rhythm, spelling or word length.

Most younger people -- the group used to draw stark comparisons with persons in their 60s or older -- are also more likely to still be actively engaged in active memorization either in school or at work. The older person, in contrast, is usually further removed from the requirement of committing to memory large amounts of data.

If an older person is trained in more efficient methods, memory "production deficiencies" can be overcome or reversed. Among the strategies that can enhance memory performance in the elderly are the use of visual imagery or the generating of a story about what is being memorized. Although the final result of these methods rarely brings the older person to the level of a younger person trained in the same techniques, it's often good enough to make up for the more glaring memory failures.

If older people are capable, after some instruction, of carrying out effective processing operations that improve their memory, why do they fail to do so spontaneously? Memory experts postulate that older people possess fewer "processing resources" -- in simple English, raw energy -- to drive their mental operations. Not surprisingly, as energy diminishes, difficult, novel and challenging memory tasks are the first to suffer; familiar operations suffer the least.

Most of the evidence that memory is impaired in elderly people is based on studies comparing 70-year-olds with 20-year-olds. Although these studies have usually tried to control for education and other variables, it simply doesn't make sense to draw comparisons that ignore the significant differences in lifestyle, attitude and experience between someone raised in the 1960s and someone raised in the 1920s. Even something like cooperating in a psychological test of memory comes much easier for a 20-year-old who has grown up in an era of surveys, tests and psychological profiles. To an older person, these tests may not seem worth the time and effort involved.

Put at its bluntest, why would a person at age 70, man or woman, care a whit about such an artificial and meaningless task as memorizing a list of nonsense syllables?

Indeed, many "memory failures" among the aged often appear less dramatic when the assigned memory tasks are made more meaningful. As an example, if an older person is asked to fit each word to be memorized into a sentence, subsequent recall improves. The same thing happens if the older person is tested not for simple recall (who was the 16th President of the United States?) but for "recognition memory": selecting the correct answer from a list (George Washington, Herbert Hoover, Abraham Lincoln, Ulysses S. Grant).

"Old subjects perform certain types of mental operations inefficiently, unless the operations are induced and guided by the task, by specific instructions, or by other supportive aspects of the current environment," says psychologist and memory expert I. M. Craik. "For instance, most older people are quite poor at describing a route through city streets, although the same people can find their own way perfectly well. In the latter situation, the context provides 'support and guidance' with respect to each successive turn and decision."

Also, older people are often more efficient. University of Missouri researchers, for example, discovered that older typists have slower reaction times. But they make up for their slowness by looking further ahead in the text. In another study, older waiters were found to keep pace with younger ones because they accomplish more with each visit to a table. Both groups -- although equally efficient when compared to their younger co-workers -- could be expected to exhibit selective "failures" of memory as result of their seniority. But they have learned to make up for these memory "failures" by novel methods of processing information that are less reliant on brute memory.

Rather than representing a net loss across the board, the memory performance of an older person is often comparable in given areas to that of someone many years younger.

The retention of ideas, the remembrance of the contents of meaningful paragraphs, the recall of words broken down into categories -- in these areas older persons score about as well as younger subjects.

Equal in importance to the performance of the elderly on memory testing are the attitudes that the majority of us hold regarding memory. Ours is a culture that highly values in our leaders the capacity to spill out on command vast numbers of facts and figures. But despite our infatuation with leaders who have the gift of ready recall, there is no evidence that memory is related in any direct way to leadership, wisdom or good judgment. In fact there is good reason to suspect that brute memory -- the ability to summon up on command and at short notice precise facts and figures -- has only a tenuous relationship to these qualities.

Consider, for instance, that most of our Supreme Court justices, many of our physicians and college professors -- and now our president -- are at an age when some degree of memory "failure" is almost inevitable. If our criteria for their effectiveness are limited to their capacity to hold forth spontaneously with prodigious displays of factual knowledge, then many of these people may have to be replaced.

The "wisdom" and "experience" of the older person -- if such qualities are related at all to memory -- have more to do with recognition memory than free recall.

For instance, a skilled physician may know on the basis of experience when an operation is necessary and when it is not. She recognizes the critical element in her patient's symptoms and knows what to do. But she may come off second best to the resident 30 years her junior who can rattle off "from memory" large numbers of facts about the patient's illness but lacks the experience to recognize which of the many categories of raw memorized information the patient fits into.

And what about the skilled elder attorney who is called in by his younger colleagues in order to listen and evaluate the legal consequences of what a client is telling them? His capacity to appraise the situation and how well the case is likely to go in court often has very little to do with the summoning up of references and citations from previous cases. Rather, it's the senior lawyer's wisdom and experience, indeed his "gut feelings" about the case that oftentimes contribute the most. If memory is involved here at all, it's recognition memory: his ability to recognize the important elements in a case because he's seen them before, although he can't always say when or where.

When we consider the case of Ronald Reagan, America's foremost septuagenarian, we have to put him into context. Has he changed? The criticism is made that his communications are low in specific information and high in anecdote. But they have been for years. This, apparently, is his personal style. Or, some say, he misses the point of a question at press conferences. Since his days as an actor, he has always done better when working from a script. Briefed on what to say beforehand, he does fine. Even in instances where he seems to stray from the point, he generally comes back to it, as opposed to the "circumstantiality" of the senile person who wanders from the point and never returns.

If the president can be faulted for his mental performance, it is entirely likely that the flaw results not from dementia but from laziness. Many who have observed him up close say that he simply does not want to become embroiled with minutiae.

When appraising an older person's "intellectual fitness" we should bear the following points in mind. First, memory isn't the only or even the most important component of sound intellectual functioning.

Second, instead of defining memory function in the older person as "worse" than in a person many years younger, it makes more sense to think of it as, simply, different. Richard Restak, a Washington neurologist in private practice, is the author of "The Brain," which accompanied the series of the same name on public television.