I think my opinions are partially clouded by the fact ive been training older guys these past seasons and Im not entirely sure of the slowdown.
I've had my 2 MVP guards since they were both 18, have tracked every week of training minutes, and pops. I've noticed a slowdown while not significant yet its amazing how fast the 18 y/o's pop. One of my 18y/o's last season had back to back JS pops something I hadn't seen before.
I would reckon by the time your guards are almost done you will be at a competitive level where you might also have to train older guys mixed in with younger drafts as well
Each stage of this game gets harder and harder to bring in a Rookie and train him properly as your facing ever better opponents. When Lundy was a rookie he was my top player and one of the top in DV, so it wqas easy to train him. Now in D3 I have to 48 my rookies in my Cup/Scrimmage and struggle to slip them some minutes in league games as their a liability. When you get to the stage of the game your aty it only makes sense to take half/mostly finished projects and put the final touches on them as they wont hurt you and your also in the financial position to be able to afford these expensive players, something I cannot yet do.