This is a topic dear to my heart. I'm 51 and have watched my peers grow old around me. Most of them started in their 30's or 40's with statements like "Oh well I'm too old now to be trying that".
IMHO in addition to all the great stuff previously mentioned I believe maintaining a young outlook is extremely important. Don't stagnate. Take up new hobbies, try new things, create new memories. If you find your conversation consists only of relating old war stories then it's time to get out there and create some new ones. Don't hang out with 'old minded' people because they'll age you quicker than anything. I feel mentally and physically exhausted after a visit to my mother and sister because they're full of advise like "you're not as young as you used to be", "you need to slow down", "Don't over do it". Needless to say I limit those visits to once or twice a year and always hit the skate park afterwards. On the physical side I believe hitting the rock climbing gym is a great addition to one's overall fitness regime. You'll find muscles you didn't know you had there.
Inspiring to read what some of you guys are doing.