They just happen to be shit footballers. I don't think playing them at a young age was neither here nor there.
For me if you're good enough, you should be allowed to play rather than languishing in reserve football. That way, a manager can quickly ascertain if this player is worth investing in and has the mentality to handle first team football, and if answer is yes.. the being careful with minutes etc, is still relevant but at least you are aware of how good the player is and that player also knows he has a future there, has had a taste of first team football and will hopefully be motivated to break into the side as a regular.
If the player is unable to handle the step up, or the game gets to his head etc - see Januzaj.. then you get rid. Fair enough - we gave him a fair shot and he failed to grab it with both hands and he's moved on. Better to do that, rather than wait and wait and then player comes through and he isn't all that.. he's wasted years in reserves.