Mine started around 30, now that I think about this. First thing was getting injured easier in the gym. Then less time in the gym and finally a pathetic 25 mins a few times a week and skipped weeks completely . R/c hobbies then stopped. I struggled but kept trying to do car work on my muscle cars projects and such. Then sex drive and energy died, 4 caffeine pill a day habit to function started, of course relationship with the wife suffered, house work backed up. Garage and basement became huge messes of parts and junk. I have the worst lawn in the neighborhood. More or less a life of neglect, stopped buying nice cloths and dress like a slob, eat like a slob. It is hard to just now realize this has happened to me.
I'm still at the start of this but so far sex drive is better, and the urge to get back to the gym and hobbies has started to creep into my head even though the energy has not been going up that fast. I lost 8 years so far.
I assume this is the general way this happens to most everyone? I wanted to chalk it up to lifestyle choices, but I have learned that one led to the other and although I tried several times to turn it around I could not.