Seems like the energy/fatigue thing could be broken down into 5-year periods.... 50 to 55, 55 to 60, 60 to 65, and so one. During each of these periods, it seems like there are noticeable decreases of energy and increases of fatigue. As the years go by, the more noticeable it is.

The way I played with my grandchildren 10 years ago... even 5 years ago... would be impossible to do now.

Lizardlady brings up a good point that there may be some other factor than just getting older. Both my wife and one of her brothers were in their middle 50s when they were diagnosed with a thyroid problem (either underactive or overactive, forget which). Their main symptom was "being tired all the time".

My wife's father was in his early 60s when he was diagnosed with adult diabetes. His main symptom was "being tired all the time".

They had other symptoms, too, but the big one was constant fatigue. They were like anyone else and thought that they were just "getting older".