Elizabeth II is still going strong, but the maximum human lifespan isn't rising at anything like the rate of average life expectancy, which is rushing upward globally at the rate of about three months a year, mainly because of progress against premature mortality. Indeed, we may already have hit some kind of limit for maximum lifespan - perhaps because natural selection, with its strict focus on reproductive success, has no particular need to preserve genes that would keep us going to 150.

The oldest woman in the world, Besse Cooper, a retired schoolteacher in Georgia, will be 116 on Aug. 26, according to the Gerontology Research Group, an organization that studies aging issues. That's a great age, but it's a hefty six years short of the record: 122 years and 164 days, set by Jeanne Calment of France in 1997. In other words, if Mrs. Cooper can get there, Mrs. Calment's record will have stood for 21 years; if she can't, maybe longer.

That's a long time, considering that there are now nearly a half million centenarians alive in the world. That number has been going up 7% a year, but the number of those over 115 is not increasing.

If Mrs. Cooper does not take the record, there are only two other 115-year-olds alive to take on the challenge, and one of them is a man: Jiroemon Kimura, a retired postman from Kyoto. He's within seven months of beating the age record for his sex, set by Christian Mortensen, who died in 1998. But Mr. Kimura is less likely than a woman to make 122, and there are fewer women over 115 today (two) than there were in 2006 (four) or even 1997 (three).

At least two people died after their 110th birthdays in the 1800s, if you're willing to trust the birth certificates. So the increase of 12 years in maximum life expectancy during the 20th century was just one-third as large as the increase in average life expectancy during the period (36 years).

In 2002, James Vaupel of the Max Planck Institute for Demographic Research in Rostock, Germany, startled demographers by pointing out that every estimate published of the level at which average life expectancy would level out has been broken within a few years. Jay Olshansky of the University of Illinois, however, argues that since 1980 this has no longer been true for already-old people in rich countries like the U.S.: Official estimates of remaining years of life for a woman aged 65 should be revised downward.

Thanks to healthier lifestyles, more and more people are surviving into old age. But that is not incompatible with there being a sort of expiration date on human lifespan. Most scientists think the decay of the body by aging is not itself programmed by genes, but the repair mechanisms that delay decay are. In human beings, genes that help keep you alive as a parent or even grandparent have had a selective advantage through helping children thrive, but ones that keep you alive as a great-grandparent - who likely doesn't play much of a role in the well-being and survival of great-grandchildren - have probably never contributed to reproductive success.

In other words, there is perhaps no limit to the number of people who can reach 90 or 100, but getting more than a handful of people past 120 may never be possible, and 150 is probably unattainable, absent genetic engineering - even for a monarch.