Your endocrine system is basically 98% of what fitness is about. This includes your signaling hormones, which decline with age.
What are signaling hormones? These are chemicals that tell your body to build muscle, repair, sleep, etc etc.
The more of these hormones you have (as well as receptors) the better potential you have to build muscle, feel less depressed and generally become 'alive'
These cocktail of chemicals are what decides if you;re going to get jacked or fat.
We know how to hijack this shit with steroids and SARMs and such.
Tell me, when I reach my 40s, why WOULDNT I want to go on hormone replacement so I can have the endocrine profile of a 20 year old?
Older people look pruned, weak and worn out because their signaling hormones drop. They are literally lesser versions of who they are.
This is the sad fact about aging. Your hormone levels drop and your body no longer tells itself to repair and build like you were in your 20s