One of my pitchers (Phil Farrell) has a potential of 80 (currently 67) with corresponding pitching ratings.  He's pitched 152 Big League innings so far this season, and 134 last season.  He has a makeup of 98.  This season, he's pitching pretty well (1.22 whip) and last season was okay (1.63 whip).  My pitching coach this season has a pitching IQ of 89.

Anyway, after all that set-up, the pitcher hasn't had any growth in any of his pitching categories this season and only saw one category go up one point after last season.  What's up?  Has he reached his 'real' potential and he's not going to get any better?

Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Phil Farrell

Yeah, he's done improving.  He really hasn't improved in 2-3 seasons.   8 year pros seldom improve.

So, how normal is it for a player to not get anywhere near his stated potential?

Very.    Projections are not 100% nor are they guarantees.   In theory, perfect development will get a player to his max potential(which still isn't the projection you see because projections are a "best guess").   I don't think "perfect development" can be achieved.

For the most part, players stop developing 4-5 seasons into their careers.   You may get a point here and there but, at that point, he is what he is.

Is that the same for a player whose drafted at 18 and one who is drafted at 21?  Or does the 21 year old only have 2-3 seasons of expected growth ahead of him?

Pretty much the same.  But the 18 year should have more dramatic development assuming, of course, their max is the same.

Thanks.

Your scouting will have something to do with how accurate your predictions are as well.  If your advanced scouting budget is 5 the predictions will be way off while if you budget 20 they will normally be closer to reality.