I cant remember where (it could have been in Crossover) but Bones details some of the work he's had done - organ replacement etc. Theoretically, if you keep sorting out the things that may kill you, you could just keep on living.
Something similar happens in Kim Stanley Robinsons Mars trilogy, but they find new problems with diseases occurring that have never been known before, specific to the super-old.
There will probably always be some form of body failure to deal with.
I think that was in The Return. I think Bones himself commented that there was very little of the original him left.