You can't be sure where it will end for kids. The fact you are talented and above generation in U18, doesn't mean that you will be the first team or even squad player in a year or two. It is weird age for those kids, being 17 or 18, with some money in your pocket and potential to become star, will bring you these days even more problems than decade or two ago. So, they need to have right mentality, right attitude, and normal people around them.
On the other hand, honestly, in terms of football quality, boys today are much better than kids you are talking about. Deahli, Cole, Rothwell, Goss, Thorpe, have been good in our team, more like stars in pretty average teams. Ravel obviously attitude problem, he could be a star with different mentality and surrounding. Januzaj, Wilson, Macheda...well, those three I think had excellent situation to succeed, which means to become first team players, but obviously did not work hard and could cope with pressure on the final step. With Fletcher attitude and working habits, they would be our players for 10 years or so. Januzaj still made a decent career, Wilson and Macheda failed. King had good career, with 5 PL seasons and like 50 goals, it is decent. Not United level, similar to Januzaj, but more than decent player.
With Garner, I don't think the problem is attitude and doesn't look like it will ever be. I think it is more experience, and waiting for the right moment for him to develop. He is somewehere in the limbo between first team and U23. Too good for U23, and still not good enough for starting lineup in PL game. Looks to me that Ole will use him whenever there is opportunity in League Cup and FA Cup, Europe maybe, as he already proved he can play on that level. Decision making is extremely important in that position, and he still use risky passes that against top side would be punished. He is close enough I think. When you consider that during preseason he was just around the team, missing a few games, than moved to reserves in the final stage. He made that step, he needs to make another one and have patience.