Improvements in medicine have led to less injuries and quicker recover times.
Lets just give you some dates for this one: Kobe Bryant-1996, Kevin Garnett-1995, Tim Duncan-1997, Dirk Nowitzki-1998, Ray Allen-1996, Jason Kidd-1994, Paul Pierce-1998. Those are the years these guys started their NBA careers and all have made significant contributions to their teams this season.
Modern medicine, along with players having much better eating habits and better training techniques, have stretched out the primes and total career years for many NBA players. We'd be hard pressed to believe Kobe would still be as incredible as he was this year had he not had those distinct advantages from being a part of this generation. Who knows what could have been if say Larry Bird could have had better treatment for his back, or Patrick Ewing was able to recover from some of his injuries.