Mitchel Lichtman has part 1 of an article up at THT, that does an extensive examination of when baseball players peak.
Lichtman takes a separate look at pre-1980 and post-1980 players, and finds that while the peak for post-1980 players is slightly later, and their declines more gradual, modern players still peak, on average, around age 28.
Bill James came in years ago with the conclusion that players peak around 27, while conventional wisdom has generally held that it is later...James incorporated that into his Fourth Noble Truth:
Ballplayers, as a group, reach their peak value much earlier and decline much more rapidly than people believe.
There's been a certain amount of speculation that, well, that was true back then, but in the modern era, the idea that players peak in their late-20s is outdated, and players generally peak later. Lichtman's study seems to suggest that while players peak slightly later, the idea that a player's peak runs through his early-30s is not supported by the data.