Author(s): Michael Eisenstein 
[illus. 1] On any weekday morning, you might catch Irving Kahn heading into his office in Manhattan, where he works as an investor and financial analyst -- seemingly unremarkable, except for the fact that he has been in the business more or less continuously since 1928.
The 106-year-old Kahn is one of many who have managed to live well into their eleventh decade with mental faculties intact and in surprisingly good health -- and researchers into ageing have taken notice. Thomas Perls, a gerontologist at Boston University in Massachusetts and director of the New England Centenarian study, recalls an early encounter with two centenarians that challenged his expectation that the remarkably old would be remarkably unhealthy. While he was training at a rehabilitation centre, Perls saw one centenarian "out and about playing piano for everybody", while another -- a retired tailor -- "was in occupational therapy mending people's clothes and teaching other people how to sew".[illus. 2]
But the data increasingly suggest that people who reach such ripe old ages are getting a biological helping hand (see 'Disease delay and genetics'). For example, recent research from Perls supports a hypothesis known as 'compression of morbidity', in which individuals whose lifespan is considerably longer than average (at least 100 years old) tend to stay healthy for longer, with delayed onset of age-associated diseases such as cancer and cardiovascular disease . "These diseases don't appear until roughly the last 5% of their lives," says Perls. If this is the case, exploring extreme longevity could provide insights into the foundations of many common diseases -- and into new weapons with which to fight them.
Armour against ageing
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 factors . 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...