Physically its one thing to continue playing. Another huge factor is what would he be going through mentally. Kobe himself said that as soon as he couldnt compete at the highest level the way he wanted to compete he'd leave. Didnt wanna hang around and just be a guy in the league. paraphrasing but i thinks thats what he was trying to convey.
kobe tore his achilles though and just zapped him
only thing as you get older risk of injury increases look at karl malone, dude played 19 and a half years without a injury until he suffered one that ultimately ended his career