Can one male 73-year-old offer some comments about another such who is about to be elected to a second term as president of the United States? We're not all alike, we 73-year-olds, in physical stamina, mental ability and by a lot of other measures. But one 73er, perhaps, has a better feel for another and for his prospects than do younger voters, as the majority of you out there are.

My wife's quick summation of the second presidential debate was simple: Walter Mondale, she said, was "dull," and Ronald Reagan was "dumb." Mondale surely was dull, however well informed he was on many a topic. President Reagan, however, was, to me, not dumb but -- well, 73 years old.

Lou Cannon, the Reagan biographer, wrote in The Post that "those near the president find no sign that he has the old fire in the belly . . ." That I can understand. It's not a matter of being "dumb," of not knowing or understanding this or that complex subject in any detail. It's a matter of limited physical energy, despite various forms of exercise; it's a matter of limited mental energy, despite continued use of one's brain.

For over half a century this journalist has been writing stories on a typewriter, putting down key by key, word by word, sentences that form in the mind as one thinks through a subject or explains a thought. It becomes reflexive, a skill honed by practice, to the point of being almost effortless. So, I am sure, it has been with Reagan, his speech making, his decision making, his acting and in recent years the combination of them before television cameras.

At some point in life, however, you begin to realize that it doesn't come as easily as it used to. Reagan still can do it off the TelePrompter just about perfectly, and he can still come up with those effective one-liners, just as he did in the second debate in turning the age issue on his rival. But if he's left on his own for 90 minutes out there behind the podium, no matter what the preparation, his internal clock slowdown begins to show. And not just in tangled syntax or never completed sentences. We 73-year-olds simply don't think as fast as we used to; it's more of an effort when we try.

Of course there are many exceptions, and we all can name some. Winston Churchill and Konrad Adenauer often are cited for their leadership of Britain and Germany when they were long past Reagan's age -- but, in reality, both were in poor shape those final years. Reagan may be in better shape than I am (doubtless he can do more pushups), but one 73- year-old can see those telltale signs of slowing down in another.

Many of us can remember a grandfather who seemed to spend most of his time telling tales of long ago. The reason, I now know, is simple: memories fade, but in a perverse way. I can recall names, places, facts, figures from dec. What's increasingly difficult is remembering who said what yesterday, and to whom and why -- in short, grappling with today's world. Neither of us has reached the point, however, of the writer well into his 80s who said the only way to be sure he'd already brushed his teeth was to feel whether his toothbrush was wet.

It doesn't make any difference, really, if I call one grandchild by the name of another. But it could make a difference if a president can't remember -- say, during a crisis -- exactly what it was that his opposite number in Moscow was hinting in his last message. Sure, aides are all about in order to help remind. But to fathom the other fellow's remark, to think it through, to progress logically, to understand not just one's own preconceived notion of the other side but what truly is most likely to be going through their minds, this takes both physical and mental energy, especially at 73 -- and 74 and 75 and 76 and 77 and at nearly 78, as Reagan would be at the end of a second term.

It's easier to fall back on aides, staff, old friends, clich,es, harder to widen the circle for fresh input. I take it that is the sort of thing Cannon means when he writes of Reagan's "passivity" and when he says that presidential aides are concerned that he "might not have any plans" for a second term. Everybody -- at least everybody in Washington who pays attention -- knows that this is a riven administration with much internal fighting over issues from environment to arms control, that there is more mediocrity in high places than usually is the case and that this president hates to make what writers like to call those hard decisions. There surely will be more of such avoidance, such drift in a second term. A 73-year-old is not about to take on the kinds of fights he can avoid; it will be increasingly so at 74, 75, 76 and 77.

The slowing down of a human being is called senility -- "mental and physical deterioration with old age," my dictonary says. We 73-year-olds don't care for the word; it reminds us too much of friends who have slipped into various forms of mental vacuity. So we try to exercise our brains as well as our bodies. Being president requires that, but the protective cocoon of the White House is always there to shield or hide or minimize tell-tale signs.

Modern medicine has done marvels for millions of us 73ers. So, too, it currently is controlling the 56- year-old Mondale's otherwise possibly dangerous high blood pressure, just as it controls mine. But the human body does wear down, including a president's. Woodrow Wilson and Franklin Roosevelt were victims of the office's demands; Wilson died at 67 and FDR at only 63, although today's medicine very likely could have extended both their lives.

When the 25th amendment on presidential succession was being shaped two decades ago, there was much discussion of how to deal with presidential "disability," one form of which might be a disabling stroke, as in Wilson's case, or even acute senility. One provision written into the Constitution allows a president, on his own volition, to step aside, even temporarily, if he decides he is "unable" to do his job.

Another provision allows a vice president and a majority of either the Cabinet or of some other body established by Congress (none such has been established) to conclude that a president "is unable to discharge the powers and duties of his office." In such a case the vice president "shall immediately assume the powers and duties of the office as Acting President." There is a further complex provision that allows a president to regain his office, if he believes he has recovered, although Congress, by a two-thirds vote of both houses, could disagree and prevent his return to the Oval Office.

It's obvious that if any such situation ever arises it would create such a national crisis as to make our two presidential impeachment proceedings pale in comparision. I