there are stages to physical development. they've been mapped for a few years now.
peaking and refining are different but essentially and in a nutshell:
1-18 = bone and tissue development along with internal organs and bowels. everything is growing and tere are massive hormonal fluxes and chemical changes occuring. bone is generally soft and tissue is loose and not dense.
18-30 = bone has sealed at the joint and cartillligenous parts are now ossifying, tissue gets more dense and organs and bowels are now fully formed.
30-40 = best time to make muscle mass even more dense and add tensile strength, bone is about as strong as it is going to get.
40-60 maintain what you can and it will be harder
60+ degradation of the entire machine starts happening as oxygenated free radicals essentially rust you out from the inside.
somewhere between 72-84 most of us can expect to die.
is death a peak? depends on your perspective.