A recently published study (Medicine & Science in Sports & Exercise, 16th August 2017) looked at the change in fitness for a group of highly trained athletes over a period of more than 40 years. The group were originally tested by Jack Daniels in the run up to the 1968 Olympics (1500m silver medalist Jim Ryun, left) and were retested most recently in 2013.
The athletes in question carry out quite different exercise levels today, with some exercising a lot and some almost not at all. On the weight front, some had gained more weight than others. Not surprisingly the athletes were among the very fittest of people of their age to be tested despite it being many years since they trained seriously with markers such as resting heart rate remaining very low.
It seems that being very fit as a young adult gives you a buffer for later in life even when you stop training. Think of it as making fitness deposits in the bank of personal wellbeing. You can draw on this later on to ensure a higher quality of life than your peer group who were less active in their youth.