The question of how and why organisms age has teased scientists for centuries. There are myriad competing theories, from the idea that ageing is a simple wear and tear process, like the rusting of a car, to the belief that ageing and death are genetically programmed and controlled. In fact, there is no clearly defined limit to life and no single, predictable program playing itself out: different things are happening within and between tissues, and each system or organ accumulates damage at its own pace, according to the kind of insults imposed on it by daily living.

Sometime before 2020, the number of people over 65 worldwide will, for the first time, be greater than the number of 0-4 year olds; and by 2050 there are likely to be 2.5 times as many older people in the world as toddlers. Sue Armstrong tells the story of society's quest to understand ageing through the eyes of the scientists themselves as well as through the 'ordinary' people who exemplify the mysteries of ageing - from those who suffer from the premature ageing condition, Hutchinson-Gilford syndrome, to people still running marathons in their 80s.

Borrowed Time investigates such mind-boggling experiments as transfusing young blood into old rodents and research into transplanting the first human head, among many others. It explores where science is taking us and what issues are being raised from a psychological, philosophical and ethical perspective, through interviews with, and profiles of, key scientists in the field and the people who represent interesting and important aspects of ageing.