Who is more aged, a 90-year-old or a 100-year-old? And, yes, it is a trick question. The answer, according to Michael Rose of the University of California, Irvine, is neither. The 100-year-old may be older, but she is no more aged, because after about 90 years we stop ageing.
Between the ages of 15 and 90, the death rate increases exponentially, but after about 90 the death rate seems to level off. Rose believes that this "plateau in late-life mortality" is a fundamental biological feature of all animals.
Natural selection is ageist - it favours the young. A15-year-old is likely to be in good shape. Faulty genes that strike before the age of 15 have been forced down to low levels, because earlier generations of youngsters with those defects died before they could pass on their genes.
But natural selection has far less to offer the middle-aged, leaving a 50-year-old far more vulnerable to defective genes. Faulty genes that strike at age 50 continue to run rife through the population, because earlier generations of 50-year-olds passed them on to their children before they succumbed.
Ageing, according to evolutionary biologists, is simply a manifestation of this decreasing force of natural selection. But as natural selection weakens, there comes a point at which it ceases to have any effect, positive or negative; its strength falls to zero. At that point, death rates no longer increase, but nor do they decrease, they simply level off, they plateau.
"What we had not been paying attention to was the third phase of life, after reproduction, when natural selection has completely ceased to matter forever," says Rose. "So we looked at this post-reproductive phase, and lo and behold, you get plateaus. So we created fruit flies that reproduce much later. When they reproduce five times later, there is a doubling in the time to the start of the plateau," says Rose in support of his theory.