The secret of eternal youth has been unlocked by scientists in remarkable research that paves the way for a 'forever young' drug.

Lives could be longer and healthier, free from illnesses such as Alzheimer's and heart disease, with skin and hair retaining its youthful lustre.

Such a drug might allow men and women to have children naturally until they are a ripe old age.

Increasing the number of years of healthy life would greatly ease health service costs and reduce the burden on families of caring for frail relatives.

The experiments mirror the plot of the film The Curious Case of Benjamin Button, where the lead character played by Brad Pitt ages in reverse.

They were carried out by cancer doctor Ronald DePinho of Harvard University in the U.S. and detailed in the prestigious journal Nature.

He reversed the effects of aging in animals for the first time in experiments on mice.

Before treatment, their skin, brains, guts and other organs resembled those of an 80-year-old person.

Within just two months of being given a drug that switches on a key enzyme, the creatures had grown so many new cells that they had almost completely rejuvenated.

Remarkably, the male mice went from being infertile to fathering large litters.

Dr DePinho said: 'In human terms, it would be like having a 40-year-old person who looked 80-plus and reversing the effects to the levels of a 50-year-old.

'By 2025 we are going to have 1.2 billion people aged over 60, which is when you start to see cancer, Alzheimer's and cardiovascular disease.

'We are on a collision course for a significant amount of burden to society.

'This is the first time that aging has been reversed.

'This suggests that there is a point of return for aging organs that we had not previously appreciated.'

The breakthrough centres on structures called telomeres. These are tiny biological clocks that cap the ends of chromosomes, protecting them from damage.

With time, the telomeres get shorter and shorter, raising the odds of age-related diseases such as Alzheimer's. Eventually they become so short that the cells die. An enzyme called telomerase can rebuild the telomere caps but is normally switched off in the body.

Dr DePinho succeeded in shocking the enzyme back to life in mice that had prematurely aged in a way designed to mimic the human aging process.

He expected the technique to halt or slow the aging process and so was stunned to find it reversed it.

He believes it should be possible to make a pill that does the same in people.

Given in middle age, it could delay or prevent the development of Alzheimer's, heart disease and diabetes. It might even extend life.

But there are important caveats. High levels of telomerase can fuel the growth of cancers, and one drug is unlikely to smooth away all the problems of aging.

Dr DePinho told the Daily Mail: 'There are multiple mechanisms that conspire to lead to aging.

'So, although we think that telomeres are important, there are other factors that come into play.'

Dr Steven Artandi, a telomere expert at Stanford University in the U.S., described the study as 'beautiful' but cautioned that an anti-aging drug is still more than ten years away.