I have a couple rookies that I have not been playing but their ratings are still growing. How important is playing your developing players really?

If you're not playing them and they're picking up it's a point here or there. If you play them as starters they can gain up to seven or eight points a season and taper off in their early twenties.

Someone else also said that even if you don't give them starters' minutes at first, you can sit them for a season or two but once you start rolling them out they'll pick back up some of what they missed out on earlier

Playing time is what determines (ultimately) what kind of player he will be. If you give good minutes (2000+) and a category goes up 5-7 points, I consider it to be average potential and I'll probably cut the player's minutes the next season and drop them from the roster when the contract runs out. However, if you are lucky enough to find that player that grows in key categories by 10+ points (like shooting) or has massive increases in spd, str, agi, sta, then you have a potential stud on your hands that could touch 75-80+ OVR when they top out. The only way you'll know is to give that new young one serious PT in their first season.

For me, unless I'm just filling a backup spot like a 2nd GK, I only sign 18yr old players with the intention of playing them to see what they got. The only exception is if I think my team is capable of winning the league title. Then, I'm probably not igning young players or I'll do what damag says and try to pick back up some of what they missed out on in future seasons.