While there have been baseball players that have stuck around into their mid 40's they are the exception rather than the rule. The reason isn't because they don't know the game or desire to still play, it is simply because they cannot keep up with the younger players. All aberrations aside, it is a young mans game, always has been, and always will be.
It is easy for people that don't play the game on the professional level to say baseball players aren't athletes, and certainly there are some out of shape loads of flesh in the game, but baseball players are athletes and those out of shape loads don't generally hang around for long. Baseball by nature is a vgame that forces the body to do unnatural things. Throwing a baseball overhand is not natural to the body which is why we see so many players develop arm problems and we aren't just talking about pitchers. Catchers take more abuse to the body than anyone on the field which is why so few can remain productive at the position full time much past their thirty sixth birthday.
The human body breaks down with age. You simply cannot remain young forever. As we age it takes longer to get over mmuscle fatigue, harder to stay in shape, and quite honestly harder to keep the high level of mental focus and intensity to continue competing at a peak level. If it was true that players could keep up with the kids twenty five years younger than them as a rule (Without a friendly visit to the Performance enhancing supply) everyone would be doing it to keep the gravy train rolling along.
Aside from guys like Satchel page who played at fifty nine- albeit a staged exhibition, you won't find anyone that really played regularly past forty seven aside from Julio Franco and even he was primarily used as a pinch hitter and emergencey replacement. As a matter of fact aside from junkerball pitchers like Phil Niekro and Jamie Moyer who is still hanging around to mixed results very few players stick around nuch past their forty third birthday. You can see even for a rubber armed pitcher playing every five days at that age is just too hard. Even the great Nolan Ryan had to take the express off the rails.
People can argue all they want about Bonds and Clemens getting it done and even getting better as they aged but we all have a pretty good idea how that happened. If that is what it takes just to stay competitive it's an indication as strong as they get players are not predisposed to being able to compete at that level with success. Like it or not I'll take my chances with an eighteen year old kid over a forty three year old, even a forty three year old legend in the game of baseball anyday.