I think the problem is that when you look at many of the past players who came through at a young age and made a real impact at the club (Fowler, Owen, McManaman, Gerrard) they looked special from the very begining, there was very little settling period and what they produced on the pitch was apparent from early on. With Sterling we are yet to see that.
He showed glimpses of his talent last season though at the age of 17/18, no one could tell me that there isn't a lot to work with, if Sterling was playing on loan to a team in the championship and ripping it up like he most probably would if he dropped down a level, no one would be doubting his talent, theres been calls for us to buy the likes of Chalobah and Hughes on here and neither of them are as proven as Sterling is, and it shows when they join up with the u21s.
Mcmanaman broke through at 19, Sterling is still 18, a lot can change in a year in terms of a players development, the likes of Barkley are current examples of this, Suso is a fitting example as well, last year he looked lost in many games, this year he is one of the best young players in La Liga. Not surprising to me because anyone with two eyes could see the talent was there, same thing with Sterling its now just about him trying to find some consistency and make a meaningful impact in the final third, but that will come with time and the more games he plays.