My experience is that it seems to depend a lot on which rating you look at. For position players, for example, I think experience is the key guide to hitting rating development, but age seems to play a bigger role in fielding development. For example, I have a prospect with one team who has a SS-only quality bat. However, by the time his bat stopped developing, he had only marginal 3B defense. However, he was an 18-year-old IFA when signed, so his defense has continued to progress rapidly through his 22-year-old season, and he is now approaching SS defense, which could make him a BL regular.
I'm not exactly sure how this applies to pitchers.