Going for a brisk 20-minute walk every day could add seven years to your life, scientists say.
In the first trial of its kind, researchers discovered that modest exercise in middle age has an anti-ageing effect on the body's cells which could extend life expectancy.
Even couch potatoes who have spent a lifetime avoiding the gym can benefit.
The study found that walking at a brisk speed where you can still speak, but not so slow that you can sing, was most beneficial to life expectancy.
Professor Sanjay Sharma, of St George's Hospital in London, said moderate exercise could halve the risk of dying from a heart attack for people in their 50s and 60s.
"We know exercise has multiple benefits on health risks, but it's more than the sum of its parts - it has an anti-ageing effect as well," he said.
"This study suggests that when people exercise regularly, they may be able to retard the process of ageing.
"We may never avoid becoming completely old, but we may delay the time we become old. We may look younger when we're 70 and may live into our 90s."
Professor Sharma added: "Exercise buys you three to seven additional years of life. It is an anti-depressant, it improves cognitive function and there is now evidence that it may retard the onset of dementia."
Several studies have analysed the statistical link between exercise and health, but they were merely observational - looking at large groups of people and calculating how many of those who exercised regularly went on to have heart attacks or strokes.
The new research is the first to look at the impact of physical activity on the body's cells, with the researchers finding that exercise protects them.
The study, carried out by Saarbrucken University in Germany and presented at last night's European Society for Cardiology congress in London, involved a group of healthy, non-smoking people.
It looked at levels of the enzyme telomerase, which plays a critical part in the control of biological ageing, and levels of the senescence marker 'p16' - a marker of cellular ageing in the blood.
Levels of telomerase drop off as we age, meaning that after a certain point it can no longer repair DNA. In the study, the 69 volunteers - aged between 30 and 60 who did not usually exercise - were divided into four groups, with one group having no change in their inactive lifestyle.
The remainder did three training sessions of 45 minutes a week - a total of 135 minutes a week - for six months. If spread out across the week, it would equate to just under 20 minutes every day.
The different exercise regimes included using a treadmill for aerobic endurance, high-intensive training for short intervals, and strength training. Markers for telomerase activity linked to ageing improved in all three training groups.
But the low-intensity endurance exercise had the biggest effect, with a four to five-fold increase. Professor Christi Deaton, of Cambridge University, said: "The study... helps us understand the process of cellular ageing, as that's what drives our organ system and body ageing, and the effects physical activity can have on the cellular level.
"The more active you are, and it doesn't matter when you start, the more benefit you are going to have."
Previous research has found that regular exercise is linked to a lower risk of death and disability, compared with people leading sedentary lives, and boosting physical activity levels can do as much good for health as giving up smoking.
In the UK, people are advised to do 150 minutes of moderate activity every week, such as gardening, dancing or brisk walking, or 75 minutes of vigorous exercise, including sport, running or aerobics. But three out of four Britons fail to achieve this.
Professor Sharma said: "If you know that something is 20 minutes away, try to walk it if you've got time and not take the bus.
"People of any age can benefit, including those with health problems. People with a heart condition shouldn't run but walk to a point where they can still speak - but they shouldn't be able to sing. Following these simple directions is essential, considering our sedentary lifestyles."
People who start exercising at the age of 70 are less likely to go on to develop atrial fibrillation, a heart rhythm disturbance which causes strokes.
Professor Peter Weissberg, of the British Heart Foundation, said the new research was "interesting science which may offer a clue to the mechanism behind the benefits of exercise."
He added: "We have hard evidence from many studies showing exercise is good for the heart. Regardless of the underlying process, however, everyone should try to do 150 minutes of exercise a week to help them stay heart-healthy."
Source: Daily Mail