It is just Age, and decades of stress from gear. Our Bodies have to take a beating from all the food, gear, hard lifting, all the stress from work, family, etc. We dance with the devil, and eventually he just will collect his due, it is what it is.
We just can't stay huge until we are 90. We can stay in great shape, but being a jacked, very lean 250 lbs and getting over 80 is just not in the cards for us Humans.