It all depends on the coaching staff at first team level, in particular the manager, whose decision will be key.
Over the years watching football I've noticed that different managers will make different choices when it comes to promoting youth. Van Gaal for example was a coach who believed in youth hence promoting Rashford, straight into the first team by, by-passing the reserves, when I didn't believe he was ready for such a big jump. He also did the same at Ajax where he promoted Clarence Seedorf straight into the first 11 at age 16. Had the manager been LVGs successor (Mourinho) or his predecessor (Beenhakker) at United and Ajax at the time, I don't believe either manager would've promoted 18 year old Rashford or 16 year old Seedorf.
So it all depends on Solskjaer and how he sees things. Because some managers believe in youth talent more than others and are willing to take the risk with youth a lot earlier than others too.
So like I said it depends on Solskjaer and also the player to some extent.