The impact of knowledge.
From the beginning until 2005, most teams and players in most years didn't really know what made a good hitter. Sure, they thought they knew, but in reality they were more in a fog than they realized. Players were thrown into the big leagues and natural selection took its course. Those that happened to develop the traits of a better hitter at the big league level survived, and the sample in general showed improvement based on natural learning until the late 20's (age).
2006 to the present, modern knowledge of developmental factors and hitting skill factors have wiped out this big league natural selection process. Most teams pretty much know what makes a good hitter. Young player development is streamlined and the natural selection process has shifted more to the minor leagues. Players now reach the big leagues as finished products, in general. We don't see nearly as many players throughout their 20's "learning" how to be better hitters. There's nothing for them to figure out on the field. Lots of players still do, of course, but the sample in general shows no improvement from learning.
The physical peak of natural athleticism has and always will be something around 20 years old. This should be obvious.
Increased knowledge has removed the foggy learning period. Modern baseball now explicitly knows from the get-go what old school baseball used to inexplicitly figure out via a slower, natural process.
(this very probably is not true)