Nature Supplement on Aging
Much of the seminal work in assessing genetic contributions to healthy ageing in the general population has been done in Scandinavia, where political peace and a strong societal infrastructure have minimized the external forces that prematurely shorten life elsewhere. “Over the past 100 years, we've basically had 'laboratory conditions' for humans,” jokes Kaare Christensen, a genetic epidemiologist specializing in human ageing at the University of Southern Denmark in Odense. From studies of fraternal and identical twins, Christensen has found that roughly 25% of longevity is attributable to hereditary factors2. Furthermore, he suspects there is a clear age dependency for this genetic contribution. “Before age 60, genetic factors are not that important in the cohorts that we have studied,” says Christensen, “but after age 60 their impact increases, and seems to get strongest at the very highest ages.” In other words, a healthy lifestyle and environment are the key determinants of whether most people will reach their seventh decade, but after that it's increasingly down to their genes.
However, a healthy lifestyle might not be mandatory for everybody. Many specialists in ageing now believe that the extremely old possess beneficial genetic variants that protect them against the vicissitudes of ageing throughout life. It is only beyond a certain age — as the health of less-fortunate people begins to decline — that these variants become apparent.