From his recent comments, it seems
Mourinho's way of promoting youth is to introduce them to a team with controlled setting (at least defensive stability is there, and the first eleven should be strong enough to get good results, meaning some first team players have to play).
Like today's games against CPalace, the team comprises of Pogba is a total domination, with Lingard providing the spice (thankfully he did), Rooney may contribute a goal (guess not) and defensive wall of Jones and Bailly.
Funny how he's against Bailly subbing off (Next time then Willock). Hence it's okay to play 'em lot. Or in last two games, the team is okay enough to get a fair enough result of draw at least, so just one youth to be promoted ~ Tuanzebe and he did fine against Arsenal and Spurs.
LVG's way is more of a
sink or swim ~ throw them on in games, and if they play well, stay with the team (Rashford, TFM & CBJ, and for some times McNair, Wilson, Varela, Blackett & Januzaj), but if they fail.. out. (Keane, Love, etc, and the players who played well for some times). Injuries to first team players would also open the door for the kids, depending on their 'best' positions.
SAF's way is more dependent on competitions, scorelines, and sometimes the risk approach. He would definitely play them in the Micky Mouse cup, and if otherwise, would sub them on in games if the scoreline is convincing enough, leading by 3 or 2 goals convincingly eg. Pogba's debut. If the first team is suffering from injuries, he may even play them but make sure to play with strong players too, like Evans with Ferdinand, Fletcher with Keane, or Rafael at RB. There is partial risk there. Also, there's the classic Fergie's Gamble
when we're chasing games and in need of a goal to win or draw, throw some goalscorer kids eg. Macheda, Gibson, etc.