All the research I've read suggests that baseball hitters most often peak between 26 and 27, maintain close to that level until about 30, and then start to slowly decline. I've never seen any research that suggests that baseball hitters peak at 30, which is why I was curious about your source. Well, I think it's a hotly debated topic in Baseball (and athletics as a whole) which is why I said some would argue for 30 as a prime age. There have been studies that suggest that 28-32 is a prime age range for baseball players. Bill James wrote about this in "Looking for Prime." Conversely, there are others such as Billy Beane who believe that 25-29 is the prime age group. Of course, it could be argued that the 25-29 age group doesn't account for late bloomers. In either case, there will always be individuals to peak outside of the normal timeline. I think Beane is also considering production for the dollar - once players hit arbitration and then free agency, their production becomes much more expensive - hence by 29 a player is past his prime in production per dollar though not necessarily in production alone.