Pique was 17 when we signed him. That is just a year younger than Ronaldo and Rooney (at the time we signed them). In that aspect, he is product of our academy around as much as them (or Cesc/RVP at Arsenal). Rossi was 17 too and spent one year in academy. Pogba was a couple of years younger though, so likely we had an important part on developing him.
Still, the role of the academy is to produce players for the first team. Ours hasn't been doing so. We have produced Fletch, O'Shea and Brown who have been quite good squad players (in Fletch's case he was awful in the first few years, but in the last couple of years he reached almost Carrick's level). That is it. We produced a few average ones in Welbeck and Evans who were more shit than good for most part and then we get rid of them, neither of them exactly proving to have been that great, and a few very shit ones like Cleverley, Gibson, Eagles etc.
So yep, since the class of 92, we have produced 3-5 good squad players, and no world class player (or a level below that), which in my opinion is quite atrocious. On the other hand we poached 2 players with world class potential and they left because of lack of chances given, and an another great players who left for the same reason. While I could understand Rossi (we had many great attacking players), not giving chances to Evans and Pogba wasn't very nice to do.