It's not just the muscles at that age you'll find out. When you're older you slow internally so the tendons and joints start to not repair as fast so make sure you avoid beta carotien and stick to the old vitamin roster since beta carotein uses the vitamin A receptor and vitamin A is highly needed for tendon repair regrowth and that will slow your physical acitivity or result in sudden injuries which is a training loss. So avoid processed shit and stay natural high fat high meat low grain and your body will respond like a machine for you.