Taking care of sleep, training and diet and my body is in the best shape and functionality it has ever been and thats during the time when it stereotypically starts to break down.

Folks you gotta take care of your body. We all have to do basic maintenance on our bodies throughout life.
The most basic resistance and cardio training, actually consistent and with some intensity. Controlling most of the diet, mostly just home cooked foods, nothing fancy even needed. Proper sleep hygiene.

All of those will go a long way to keep our bodies functional, but only if done consistently and for as long as you want to remain functional.