President Reagan, 75 years old today, is a prime example of a newly recognized and growing group in society -- the "young old."

Although his vigor -- riding horses and clearing brush at the ranch, pumping iron at the White House -- has been viewed with a measure of surprise and awe, researchers have found in recent studies that he may not be so unusual, not the rare exception that many have believed.

In Reagan, the country has a president four years older than the previous oldest holder of the office, Dwight D. Eisenhower. While selling his budget package at two agencies yesterday, Reagan took time to joke about his age.

He told a Treasury Department audience he prefers to think of his birthday as the "36th anniversary of my 39th" birthday. "A few more of these and I'll be just about due for a midlife crisis," he added.

"In fact, I'm thinking about a career change," Reagan said to laughter, "drop this political business and see if I can't do something different like radio or the movies."

Reagan is clearly one of the young old, those over 65 but not over 85, most of whom are healthy and capable. In the past, society has characterized older people as senile, weak, sick. The meaning of "old" was to be diminished.

In 1952, Alben W. Barkley was a well-liked vice president, and he wanted to succeed Harry S Truman as president. But labor leaders of considerable influence in the Democratic Party vetoed Barkley's candidacy. He was 74, and "too old," they declared.

Barkley, working long hours in the campaign, was angered by the age issue, but others, including the editorial page of The Washington Post, said the election of an aged man to the presidency "would be too much of a risk to run."

Now researchers say we must rethink the meaning of age. Those who are old are mostly not disabled by disease or mental failure. Those who do have disease are not that way because they are old.

Dr. T. Franklin Williams, director of the National Institute of Aging, related a story on this crucial point about one of the pioneers in the field of gerontology. The aged researcher went to his doctor with a complaint of pain in his right knee. The doctor told him there was nothing to be done: "It's just old age."

But, the eminent patient protested, "My left knee is the same age as my right. Why is it not suffering from old age as well?"

Hearts with no disease, at 40 or 70 years, pump the same. Other physical measures also show that, as long as disease does not intervene, physical capability is not impaired. Two-thirds of those at 75 are in good health.

Mental capacity for the old is different than that of the young, because while some quickness seems certainly lost with age, other powers are gained. For example, the vocabulary and specialized knowlege of a 75-year-old is on average greater than that of a person in his 20s, according to David Arenberg, chief of the cognition section of the National Institute of Aging.

A prime example of the prejudice that exists about the aged is what happened when the name of "senility" was changed to "Alzheimer's disease." According to Shervert H. Frazier, director of the National Institute of Mental Health, it drastically revised people's thinking about at least one aspect of aging. Senile is something a person is, but Alzheimer's disease is something one gets and can be treated for.

Like many of the young old, Reagan has had a bout with serious disease -- cancer. He has a chronic illness in the recurring polyps of the colon and growths on his nose and face. While chronically ill by the technical definition, he is not disabled. Only about a dozen days per year are "bed disability days" among his age group. About 20 percent have short hospital stays in a year.

If Reagan's vigor is greater than others among the healthy aged, it is his good health habits, education and well-to-do financial status that stand out statistically in his favor.

Less than 10 percent of those his age, on average, have a college education. Only about 12 percent have jobs, but many own their homes. The median income for men his age is $6,000 a year, and for couples of that age, $10,000 per year.

The trend is not only to larger numbers of healthy old people, but to a similar boom in very sick old people, especially among those over 85, the "oldest old."

Reagan and others of his age group can expect about 10 or more years of life. Reagan seems already to have an edge on some of his peers; he was born earlier than the other living presidents -- Richard M. Nixon, Jimmy Carter and Gerald R. Ford. He also was born before John F. Kennedy. And many consider Reagan the easiest-going president since Eisenhower.