In Jonathan Swift's 'Gulliver's Travels,' Gulliver encounters a small group of immortals, the struldbrugs. 'Those excellent struldbrugs,' exclaims Gulliver, 'who, being born exempt from that universalcalamity of human nature, have their minds free and disengaged, without the weight and depression of spirits caused by the continual apprehensions of death!'

But the fate of these immortals wasn't so simple, as Swift goes on to report. They were still subject to aging and disease, so that by 80, they were 'opinionative, peevish, covetous, morose, vain, talkative,' as well as 'incapable of friendship, and dead to all natural affection, which never descended below their grandchildren.' At 90, they lost their teeth and hair and couldn't carry on conversations.

For as long as human beings have searched for the fountain of youth, they have also feared the consequences of extended life. Today we are on the cusp of a revolution that may finally resolve that tension: Advances in medicine and biotechnology will radically increase not just our life spans but also, crucially, our health spans.

The number of people living to advanced old age is already on the rise. There are some 5.7 million Americans age 85 and older, amounting to about 1.8% of the population, according to the Census Bureau. That is projected to rise to 19 million, or 4.34% of the population, by 2050, based on current trends. The percentage of Americans 100 and older is projected to rise from 0.03% today to 0.14% of the population in 2050. That's a total of 601,000 centenarians.

But many scientists think that this is just the beginning; they are workingfuriously to make it possible for human beings to achieve Methuselah-like life spans. They are studying the aging process itself and experimenting with ways to slow it down by way of diet, drugs and genetic therapy. They are also working on new ways to replace worn-out organs