I think the difference is back then youth players would wait their turn and bide their time. They did some loan spells and got mens football, only the true greats broke into teams early (Giggs). He drip fed the likes of Beckham, Neville, Butt, Scholes into the side over a number of years.
Beckham was 19/20 by the time he started playing in the first team regularly but had been around the squad since he was 17.
Fergie had enough time to evaluate these players and realise they were ready to take over, i'm not sure Ole has had that amount of time with a lot of these lads.
He has given the likes of Greenwood, Shoretire, Williams, Elanga their first starts and plenty of minutes, i'm not sure Fergie could have done it a lot better.