Pride, vanity, an addiction to endorphins, good habits, the will to survive–all these things keep you young and going back to the gym, or the cricket pitch, or the rowing club, or the squash court, or the bike, or the neon running shoes.
Keep your body hard while others let their bodies grow soft does something to you. Rack up enough hours on the ergometer, the treadmill, or hefting those weights and you start believing you are immortal.
There are three ages of physical exercise. First, you do it for fun. Then you do it for fitness (wanting to make the most of what God gave you). And finally you do it to stay alive (wanting to watch your kids grow up).
Find a reason to keep that ticker fit and keep growing naturally young.