Have kids and compare yourself against them. It truly sobering to see how much relentless energy hey have, in body and mind (and when they hit the wall with burning out all energy, they hit hard). All the movement is just burning through all energy. I feel myself very active, but next to my 3 year old son I feel glacial and lazy.

We can reverse that, like we can revere many ways we slowly decline, just need a right mindset. Spend a really active vacation or start a new sport and lack of energy of yesterday will be gone, to certain extent. One can always do some dramatic change in lifestyle to see a dramatic change in body, weight, strength, stamina etc.

I see as people get older their mental model of how they behave and think often mimics somebody much older than their actual age, acting as if they are completely powerless to greater evil forces of muscle atrophy and weight gain. It simply ain't true but going against it is certainly harder than just complain.