He's at an age where most players are still in the low minors.
This is true.
These things are also true:
1. He is not in the low minors, and hasn't been in the low minors for some time.
2. Due to injury, mental issues, whatever - he has not shown the rate of improvement most people expected. He improved by about 3/4 of a win in terms of his batting last year if he had played the same amount as in 2012.
3. Things like defense and baserunning have already peaked for him, or will likely peak in the next year or two.
Look, if Bryce doesn't improve from being a +25 run hitter (think Evan Longoria), and settles in as an exciting but neutral baserunner and defender (I think likely) he's still a 4-win player. That's great. I think he'll be able to maintain that level of performance, in slumps and starts, just based on his pure talent.
But I also think it's fair to start wondering when these big jumps of on-field batting production are going to take place. If a player is going to become a PLAYER, you normally see the jumps early on - Trout, Stanton, etc.
There are plenty of guys like Hosmer who were "can't miss" hitting talents (.293/.334/.465 in his first season at 21 - sound familiar) who never make those huge leaps. That happens, too.
I don't know, I still think it's going to happen.