I'm pushing 40 now, and one thing I've learned is to recognize when my body is telling me "you need to back off now." I don't recover the way I used to (I can't lift 4 days a week anymore, for example, and squats and deadlifts are right out due to back problems), so I need to be careful not to overdo it. Because then I'll hurt myself and have to take a few weeks off -- and worse, I'll be hobbling in to work for a week or more like an old, broken man, and all the millennials will laugh at me :)