Two or three exercise sessions a week kept only some arteries healthy, a study of 100 people in their 60s found.

The researchers said any form of exercise reduced the risk of heart problems.

But the right amount of exercise at the right time in life could reverse the ageing of the heart and blood vessels.

The study, published in The Journal of Physiology, looked at participants' exercise history throughout their lives and measured the stiffness of their arteries.

Arteries are blood vessels which transport blood in and out of the heart and to all parts of the body.

Each one is shaped like a tube and as people age, they are prone to stiffening. An unhealthy lifestyle also causes the arteries to become blocked with fatty material.

The US researchers found that exercising two to three times a week (30 minutes per session) over a lifetime led to more youthful middle-sized arteries supplying blood to the head and neck.

However, people who exercised four to five times per week - roughly the amount recommended by NHS guidelines - had healthier large central arteries as well as healthier middle-sized ones.

The larger arteries are the ones that supply blood to the chest and abdomen.

The research did not take into account factors such as diet, social background and education in their analysis which would also have had an impact.

Dr Benjamin Levine, lead author from the Institute for Exercise and Environmental Medicine in Dallas, Texas, said: "This work is really exciting because it enables us to develop exercise programmes to keep the heart youthful and even turn back time on older hearts and blood vessels."

He said previous research had shown that waiting until 70 was "too late to reverse a heart's ageing" because it's difficult to change the structure of the heart and arteries at that stage, even with a year of training.