When you don't have a strong approach to training players (Barcelona, Ajax) or when you don't need to sell trained players to survive (90% of clubs in Eastern Europe although even Hajduk Split seem to have moved on from this approach, recently*), bringing youth players into the first team is always a throw of the dice. There's so many factors... even if they do well in a lower league, much depends on the way the team played and few clubs pay any attention to that. Jose Pekerman who won a couple of Youth World Cups with Argentina said himself that there's no way to know, even at 16 et 17, if a player is going to live up to his potential. It only takes a bad influence or an injury to get a player off-track and, on the flip side, some youngsters are more suited to a higher level.
In my opinion, the best way to go is to trust the coaches who see them on a daily basis. But even then, it's a tough thing to ask a manager to put his job on the line for the sake of a youngster who may or may not make it. You play a kid and you get thrashed and it's quite likely people won't see you as a visionary. To his credit, I think Sherwood always said positive things about Kane. Postecoglou looks the friendlier type so maybe now's the time, as confidence is a very important aspect of a young player's game.
* which, by the way, shows how fudged up is football: it's easier to make a profit on a mediocre South American player with a good/connected agent than it is to train a young player. If the kid's good but not exceptional and if you don't make it to the later rounds of whatever European competition you're in, nobody will even take a look. A weaker player with a good agent is sold in a couple of years.
