Lets stop acting like Brook isn't immortal. His DF doesn't just bring him back to life it defies all logic.
Given his situation once his spirit found his body as a skeleton the moment he went back in to it he should've died again. Brook isn't a mortal any longer. He doesn't need to eat, has no means to breathe, has no internal organs, or a brain, etc. His soul is the only thing sustaining him.
The only thing Brook has to fear is "drowning" in the sea. I don't even think dying from old age applies to him since his spirit was away from his body for so long.