There's no standard answer and varies on an individual basis (since every player is fundamentally different than a standard clone off the conveyor belt, especially in terms of personality), IMO - so a lot of factors are in play at the same time. eg. A lot of players develop slowly from a physical perspective (like Lingard), so their limit for qualifying as a young player varies from the young of someone who was ready to play in the first team at 18 because of greater physical maturity, and had already played 4-5 seasons by the age Lingard broke into the United team (so it's judged on the amount of time the player has been at the level, and the improved he's evidenced in that time-frame).
Though in general terms, positions that require greater tactical understanding of the game and relative technical mastery are naturally harder to play, so the upper limit for being considered young is raised (apart from exception to the rule type players - like Verratti and Weigl, who were consistently good at traditionally harder positions from a very young age).
Fast strikers (unless they're super raw and need to refine their play) are expected to produce at a younger age than traditional target 9 players that play higher (who have to be better at hold up play, need to have a better perception of space because they can't always dribble really fast to stretch the defense, need to run in the proper channels to disorganize the opposition defense, be physically stronger to hold the defenders off, etc).
Normal box-to-box players who can make an impact with their stamina are expected to produce at a younger age than holding midfielders or registas (who have to be sounder in terms of technique, positionally advanced to screen the defense, should know when to pass and whom to pass to, etc).
Same goes for normal centerbacks vs sweepers who're expected to be more perceptive and better on the ball, or support strikers and forwards vs classic #10 players who can act as proper orchestrators for their team and need to have better decision-making ability.
Wrt. Pogba, some of the criticism is a bit harsh. He was always an inconsistent player (oftentimes world class one game, and fundamentally poor the next) - even at Juventus, but he has slowly learned to minimize the underwhelming performances (still a work in progress), plus, there he had the supporting cast to take the pressure off him. At United, he's expected to be the leader from the get go (Juventus had Buffon, Pirlo, Vidal, Marshisio, Bonucci, Chiellini through the years - so he didn't have to lead and organize the team per se), and he's expected to drag the team out of trouble when things go awry (happens with a greater frequency than at Juventus - who were on the front foot and leading in most of their Serie A matches), and he's expected to be the face of the club. That's too big of a transition (especially when coupled with the way we asked him to be a proper two way box-to-box player through the early parts of the season), and he needs time to process all of that, and figure things out.
His fee doesn't mean anything - we overpaid by a massive margin, and not every big money signing has to perform at a World XI level from the get go - the money was more about investing in what he will be over the next 7-8 years (hopefully), than what he is right now. He needs to improve as an individual, no doubt, but he also needs a more improved supporting cast to ease the burden on him, and make sure that every time he doesn't deliver - the focus isn't too sharp since others know how to pick up the slack. For reference, after Bale's move to Madrid - Ronaldo was still the established star, plus - they had Modrić and Ramos and Benzema and Marcelo and Di María (in the form of his life), so he could be more relaxed - though even in his case, the media was too critical too soon and some of it got into his head - creating a bit of doubt, which is never a good thing.
