Longevity project by Howard Friedman and Leslie Martin is a great book you should read. I just read about physical activity level of studied individuals and how long they lived.
This is what they found:
Being active in middle age was most important to long and healthy life.
Being a childhood jock did not lead to long life if you were one of those who quit sports and slowed down a lot as you aged.
Being inactive in childhood was not a problem if you became more active as you aged.
Those who were active in youth and stayed active tended to live very long lives.
Those who were inactive in youth and became more active often did almost as well in terms of life span and sometimes equally well.
Those who were active were more likely to stay active, but if they did not, their risk of dying sooner increased substantially.
There is a powerful message in those words. And the message is, that it doesn’t matter how much exercise you did when you were really young. As soon as you stop pushing that aging body physically you will be getting old faster. And if you stop exercising and start leading sedentary lifestyle, you will most likely die sooner. Maybe a lot sooner.