As of last year Kobe didn't have any cartilage in his knee, that is an example of your body breaking down. It doesn't matter what your dna is, you don't regrow cartilage. You lose mobility in the joint, then the joint stiffens up, then you lose the ability to use that joint entirely and have to get the replacement. Kobe had an experimental procedure to help alleviate the symptoms of it, but how long will that last for him?
You see it a lot when players haven't played a full season yet, they play a full season then get injured the next year, or they "hit the wall" during the year and lose effectiveness.
NBA miles is a big thing, and it isn't just the game it's the training as well. Basketball is very high impact on your joints. Kobe is still playing at a high level, but he's lost some of is explosiveness over the years. Same is true for all players.
Kevin garnett has also kept himself in really good physical condition. He's very lean, and trim. He doesn't have a heavy build and he's in great shape. He is putting less weight on his joints by staying at a lower body weight. Kobe has lost weight twice over his career. I think he is at his lowest playing weight of his career right now, after losing weight before the olympics a few times. That is going to help his joints out a lot.
Kareem abdul jabaar played a long time, and attributed that to playing with lighter shoes. He felt that carrying the least amount of weight as possible on your frame was essential to your overall health. Kareem also kept himself at a lower body weight and in good physical condition throughout his career. You see the guys who carry a lot of weight getting all kinds of lower body injuries that nag them later in their career. It's not age, it's the cumulative effect of all those steps taken on the court, all the running and jumping.