I consider myself old, and I'm 31, but in reality, with today's medicine I believe anything under 70 is still young enough to have fun. Old for me would then begin at 70 and even then only loosely. I truly believe that you are only as old as you feel.
1. because you are overweight and out of shape
2. more to life than the pursuit of fun- this is an infantile approach to things
3. If you truly believed it , you should start acting like it.
As for OP, late 70's are when the bulk of people who havent been culled yet rapidly start loosing their fastballs physically and mentally