I suspect it has more to do with the analytics available now. When the players first come up, people have to discover their weaknesses and counter any adjustments the player makes. After two years or so, the player is likely to be unable or unwilling to make further adjustments (maybe minor tweaks), so the book on how to attack them grows while their ability to adapt to the changes often doesn't. Add to that a general reduction in speed as the player finishes filling out, and an early peak makes some sense.
The early decline may not be from degradation of skill, other than a little speed, but from the adjustments being made by the pitchers.