A brisk 25-minute walk every day can slash your risk of dying from a heart attack by half, and add seven years to your life, say researchers at Germany's Saarland University.

Even those who don't start exercising until their 70th birthday can still reap benefits, including lowering their odds of developing atrial fibrillation.

The researchers studied a group of inactive but healthy people who began an exercise program.

After six months of regular exercise, blood tests showed that all forms of exercise — aerobic, high intensity interval training, and strength training — had positive effects on the markers of aging, including the repair of old DNA.

Endurance and high intensity exercise were most beneficial.

"This study is very relevant," Sanjay Sharma, professor of cardiac diseases at St. George's University Hospitals NHS Foundation Trust in London told the Daily Mail. "It suggests that when people exercise regularly they may be able to retard the process of aging."

He advises everyone to walk at least 20 to 25 minutes every day.

"We may never avoid becoming completely old, but we may delay the time we become old," he said. "We may look younger when we're 70 and may live into our 90s."

The study helps researchers understand why physical activity has positive effects on aging, says Christi Deaton of the Cambridge Institute of Public Health.

"The more active you are, and it doesn’t matter when you start, the more benefit you are going to have," she told the Mirror.

"We recommend people who have cardiovascular disease or had myocardial infarction or heart failure to be physically active, because it’s beneficial for them; so there’s really no reason for healthy people not to exercise as well."

The study was presented at the European Society of Cardiology Congress.