For centuries scientists have been attempting to come up with an elixir of youth. Now remarkable discoveries are suggesting that ageing is something flexible that can ultimately be manipulated. Horizon meets the scientists who are attempting to piece together why we age and more vitally for all of us, what we can do to prevent it. But which theory will prevail? Does the 95-year-old woman who smokes two packets of cigarettes a day hold the clue? Do blueberries really delay signs of ageing or is it more a question of attitude? Does the real key to controlling how we age lie with a five-year-old boy with an extraordinary ageing disease or with a self-experimenting Harvard professor? Could one of these breakthroughs really see our lives extend past 120 years?