Think about what you're saying from a logical perspective. Of course there's evidence that shows that... because if you're coming straight from high school (or have that opportunity/ability) it means you were a better player or had higher potential in the first place than the guy who went four years. Nobody is staying in college because they WANT to, they're doing it because their skills necessitate it. Do you think the players who succeeded right out of high school would have been worse players or had lesser careers (outside of the obvious possible longevity difference) had they entered the league at 21 as opposed to 18?
And of course there isn't an arbitrary age where you're "ready", because everyone is different, but using that logic, why not just let in kids who are 13 if teams are willing to draft them? There's a greater chance of players being ready at 21 than 18, that's just a fact. Players generally speaking get better as you age, gain experience, and physically mature until you plateau.