The difference does not involve changes in genes, as far as is known, but changes in the human form. It shows up in several ways, from those that are well known and almost taken for granted, like greater heights and longer lives, to ones that are emerging only from comparisons of health records.

The biggest surprise emerging from the new studies is that many chronic ailments like heart disease, lung disease and arthritis are occurring an average of 10 to 25 years later than they used to. There is also less disability among older people today, according to a federal study that directly measures it. And that is not just because medical treatments like cataract surgery keep people functioning. Human bodies are simply not breaking down the way they did before.

Even the human mind seems improved. The average I.Q. has been increasing for decades, and at least one study found that a person’s chances of having dementia in old age appeared to have fallen in recent years.

The proposed reasons are as unexpected as the changes themselves. Improved medical care is only part of the explanation; studies suggest that the effects seem to have been set in motion by events early in life, even in the womb, that show up in middle and old age.

“What happens before the age of 2 has a permanent, lasting effect on your health, and that includes aging,” said Dr. David J. P. Barker, a professor of medicine at Oregon Health and Science University in Portland and a professor of epidemiology at the University of Southampton in England.

Each event can touch off others. Less cardiovascular disease, for example, can mean less dementia in old age. The reason is that cardiovascular disease can precipitate mini-strokes, which can cause dementia. Cardiovascular disease is also a suspected risk factor for Alzheimer’s disease.

The effects are not just in the United States. Large and careful studies from Finland, Britain, France, Sweden and the Netherlands all confirm that the same things have happened there; they are also beginning to show up in the underdeveloped world.