Basically like the above poster said.
Baseball has a great system for how they run their league (One many people feel other sports should adopt). Basically, if you're good enough to go pro out of high school, go. If you go to college to play you have to stay at least three years, then you can come up. Either way is fine, and lots play sooner by going to college because they can't compete in the minors yet. The minors is made up of struggling major league talent, young talent, old talent, injured talent, and any other kind of talent with a small problem in the majors. College is much easier because everyone is the same age at the least.