I have noticed that an 18 year old will gain about 3 points O/A if he just sits. Won't grow a superstar that way. If you play an 18 year old as much as he can, the average is about 7 points O/A gain, if he can play over 30 games. Some players will gain more then that in the first year, some will gain less. The ones who gain 7 or more I expect good things from. The others, depending on the skills they need and the skills they gain, they may or may not stay. I won't take an 18 year old unless I commit to using him.
Here is my rule of thumb for young players:
18 7 O/A gain
19 6 O/A gain
20 5 O.A gain
21 4 O/A gain
22 3 O/A gain
23 2 O/A gain
24, 25, 26 1 O/A gain [maybe]
by 27 they should be done growing and after 32 or 33 they start to go down. By the way, this is all my opinion.