My theories: 1. Baseball was stuck in the dark of ages of "there is a right way to do this" for a long time. Then they started hiring different kinds of leaders, and those people did some math, and figured out that young players were more athletic, better, and cheaper than older players. I think this is number one, a different type of leader has emerged that isn't in love with "proven veterans" and experience are needed to win. 2. They are cheaper than older players, and owners like money. 3. More international players? I don't know, but I would guess that the there are more international players, those players debut younger because they sign so young. 4. More willingness to promote players faster in the minors. 5. Teams are afraid pitchers will get hurt, so they promote them faster (especially RP) so they get value out of them before they are hurt. I've read this is true, but that doesn't make it true.