As I said on another thread, Fowler and Owen were playing at 17, gerrard at 18. Sterling should be getting the occasional game at his age if he's going to be big.
Did they replace proven international players when getting their chances? Downing might not be all that, but he's an England regular. I can't see some 17 year old from the same country getting the nod above him in a season where we have no European competition, and competition for 4th is tight.
I'm not saying Sterling doesn't deserve a chance, but I get the feeling that clubs take pushing younger players into the first team much more seriously than they used to because the threat of long term injury ruining the player's career is so much higher. Look at Wilshire, for instance. Great player, but it really looks like he will struggle with injuries throughout his career.
Making it hard for these youngsters to break into the first XI is necessary so that they remain focused during this time when real life can be more interesting than kicking a ball (look at that Manc kid Morrison), but I agree with Rafa that if the player has a gift he should get a taste every now and again. Pacheco had those tastes and squandered them all. Kelly (though a bit older) has taken them all (outside one poor back pass). The chances for these guys tend to come on loan, unless you're like Flanagan and have had first team thrust upon you because of mass injury to the senior players. These players can be judged on their loan period, eg Amoo who is burning up the lower leagues in the way Pacheco did not, and Shelvey who did very well at Blackpool.