I think it's pretty evident from watching a youngster after only a few games as to whether their going to make it not.
You see little glimpses of what they have to offer even if only on a small scale. Whether it's breaking the odd tackle, stepping there opponent or displaying good ball skills. There's signs there to suggest that they have the potential to become a firsteamer. Then in other cases a youngster can play twenty games plus and show very little and usually they fade into obscurity. Now of course there's always those that buck the trend but those are few and far between.
What comes with maturity is consistency. Youngsters suffer from inconsistency, again there's always exceptions to the rule but in the main they struggle to replicate good form week in week out. Instead we see them go from good game to bad game in the space of a week. It's only through maturity and experience that they're able to overcome that inconsistency and blossom into regular consistent firsteamer's.