According to most studies, the human brain stops growing in its early 20s, after which it starts to contract. (CBC)

Having just turned 60, George W. Bush submitted himself for his annual check up. White House doctors declared the U.S. president exceedingly fit for a man his age — he's an exuberant bicyclist — but they also noted he is a quarter of an inch shorter than he was just a year ago.

Should we be surprised? This, after all, is a man with the weight of the world on his shoulders. Though perhaps the diminution can be attributed to a loss of stature on the international scene.

Alas, the real reason is more prosaic. The considerable powers of the U.S. presidency notwithstanding, Bush is going through the same life process as the rest of us.

Simply put, men and women shrink as they age. The lubricants in our joints dry up, our bones lose mineral density, and even our brains become physically smaller.

And there seems to be no real way of slowing this biological deflation. Not exercise, not diet (though there is some debate here on the calcium front) and certainly not status.

One celebrity website that claims to keep tabs on the verticality of those in the public eye notes that actor Clint Eastwood and wrestler Hulk Hogan have each lost about three inches since their prime. If true, that's quite a chunk to be lopped off.

According to the most oft-quoted research on this subject, the Baltimore longitudinal study on aging, men at 70 are on average three centimetres or 1.2 inches shorter than they were in their 20s. For women the difference is five centimetres or nearly two inches.

This compression of the human body certainly picks up steam in the latter years, but the sad fact is aging is a young phenomenon.

According to most studies, the human brain stops growing in its early 20s, after which it starts to contract. Muscle strength peaks as a rule around 25. Aerobic strength or oxygen intake maxes out a couple of years later. Height loss starts around 30 in most people, which is around when bones reach their maximum density. The eyes start to go in your 40s and your intestines even begin to shrink.

Face up to it boys and girls, by 30, maybe 35 at the latest, your best years are behind you.

Height, bones and old faces

Humans shrivel up primarily because their bones get smaller as they age. After 35, bones begin to lose minerals, calcium primarily, and the problem is most acute in women.

Between 40 and 70, bone density in men falls by up to 15 per cent, though its rarely noticed in the middle years because that's when guys are packing on the poundage. The loss, though, makes the male head relatively heavier on the spine and that situation coupled with routine muscle loss contributes to the sadly familiar S-shape of the middle-aged male with his projecting tummy and the often-distinctive forward slope of the neck.

Bone density loss in women is often more pronounced, and more serious from a brittle bone perspective. (At 65, North American women have twice the rate of hip fractures as men.) But density loss in women appears to come on most strongly in middle age, during menopause, which is why many women are often prescribed calcium supplements around that time.

That calcium loss probably explains why women seem to grow shorter, particularly later in life. By 80, the average male is five centimetres or not quite two inches shorter than he was in his prime; for the average female, the mark on the kitchen doorframe would be eight centimetres or 3.15 inches less than what it was her 30s.

Bone loss can also explain why a woman's face can look older than her hubbie's, to someone's everlasting aggravation.

Scientists used to think it was loss of muscle tone and gravity that led to wrinkles and older faces, the antidote being the nip and tuck.

The more recent thinking, however, is that old faces are the result of facial bones shrinking in size and sucking in the skin and muscle around them. And the unfair bit of biology here is that women appear to lose facial bone structure at a younger age than men.

Brain size

The human brain is a marvel of 100 billion cells firing away on all fronts in a nifty electro-chemical soup. Unfortunately it stops growing not long after your teen years (parents might argue at the beginning of) and actually begins to condense in size.

The human brain shrinks about 10 per cent a decade, according to a U.S. study, though women's brains appear to shrink much less than men's. A man at 64 has a measurably smaller brain, in both weight and volume, than at 60, several studies have noted. Equivalent female brains don't seem to change much at all.

The good news, according to Australian researchers, is that size doesn't matter. Cognitive abilities such as memory, attention and speed of processing are unaffected, they say.

Cognitive decline or what's called age-related memory loss are not due to a drop-off in brain size or neuron loss, as had been previously believed. Scientists now say these seniors' moments as well as their more insidious cousins have more to do with complex chemical interactions in the brain that deteriorate over time.

Neuron loss, by the way, is also much overrated. It used to be thought they couldn't be replaced at the same clip as humans aged, that each individual lost thousands of brain cells every day. But with new technology, researchers are now able to say that the boomer brain, for example, has pretty much as many neurons as it did when the Beatles first arrived on the scene. They just may not be in all the places they were before.

Strength

Muscle strength for both men and women peaks around 25, plateaus through your 30s and then begins the quick march downhill so that by 65 you have lost about 25 per cent of what you had at your peak.

Both sexes seem to lose muscle strength at about the same rate. But because men are on average stronger, women can become limited by loss of strength at an earlier age.

Some researchers feel that static muscle strength remains pretty constant through your life and only begins to decline in your late-60s. What changes is the ability to perform complex muscle movements, even as modest as cranking, by one's late 20s. Grip strength is said to decline at the rate of almost three per cent a year after 50.

Strength and even aerobic fitness are relative of course. A fit 60-year-old can probably outperform a 25-year-old couch potato. But the 60-year-old won't have the strength, muscle flexibility or, importantly, the oxygen intake of a normal twenty-something.

Sports research has suggested that a 65-year-old who undertakes a (responsible) three-month training program may be able to develop the aerobic abilities of someone in his or her mid-40s. But once they've reached that peak, their oxygen intake starts to drop off again at pretty much the same rate as everyone else, which is exactly what appears to happen even with pro athletes in their 30s. As soon as the intense training stops, the decline sets in.

The human metabolic clock, it seems, is geared to wind down. The advantage of exercise of course is that it may be able to delay the final accounting a bit and at the very least preserve good health right up until the moment when the guy with the dark robe and blood-stained scythe turns up in your backyard.