So players barely improve after 21 or 22?

This definitely isn't the same in real life

In HBD, position players mostly develop during their first 4-5 seasons. Pitchers a little longer.

I would agree with the above with the following differences:

Most players (regardless of player type) will do the great bulk of their developing during the first 3 full seasons. They will get a few points in the 4th season and occasionally develop into their 5th season.

What I find to be true, more often than not, is players who are drafted @18 will develop a bit longer (they are the guys who will develop a bit more than usual during the 4th and 5th seasons). Players who are drafted as College seniors (age 22) will peter out development wise prior to their 3rd season. They will still develop a bit, but it'll look more alike a 4th or 5th season, even though it is their third.

Tied to development and the likelihood a player will develop more or less than the "typical" player is the Make Up rating. The higher the MU the better chance he will develop better and longer. I also find IFA's to be different. I think IFAs who are 20+ when signed do not develop much. 18 or 19 Year Old IFAs tend to develop more normally.

Just based on your opening "So players barely improve after 21 or 22?" I immediately assumed you were referencing an IFA signed at 20 years old.

One thing admin stated they wanted to do when they rejigged the scouting a couple of years ago was to make older players a bit more projectable and I think they achieved that for the most part. In one world I have 20 mil College and 22 yr old players are pretty much what you think they're going to be. In the other world I have 20 mil HS and even with 20 mil HS an 18 yr old player is much more volatile. Just. My. Considered opinion.

The important part, for our purposes, is that all these things are observable.