Barry Bonds played at a high level into his 40's. Yes, I know all about the steroids, but he did play that long. Tom Brady played (albeit football) until he was 47. There will be a very few guys who can play into their mid to late 40's, but Father Time usually catches up with them long before then.
Pete Rose by the end of his career was a spank hitter. His base hits were almost always landing in the shallow parts of the outfield. He no longer had the drive in his swing to give the ball a ride. Hitters in baseball usually fall into two categories that spell their demise. They either cannot turn on a fastball anymore or they cannot read the spin of the laces on the breaking balls. They are finished once the pitchers figure this out. The pitchers lose a few miles per hour off of their pitches and then the batters can turn on them and drive the balls almost at will.
That is what makes the guys like Pete Rose or Nolan Ryan really special.