The Hero's Journey came in for a fair amount of abuse because you can torture any story into it.
But we talked about the quest for whatever object -- like the Grail -- The Lord of the Rings is in fact a reversal of it, going to throw away something. The Hobbit is rather closer to the common form, with its aim of treasure. Then there's the quest of destruction, to track down and kill the monster. As in Beowulf, where he goes after Grendel. (If the dwarves had aimed at killing Smaug, they would have done that, too.)
In medieval times, the knight errant did not loll about the court, waiting. He fared forth, and if it wasn't on a quest, it was a quest for a quest. Knights were not meant to sit about waiting to hear about quests.
Setting out to seek your fortune in fairy tales -- well, sometimes, you were forced out, a la Snow White or Catskin, because your stepmother hates you, or your father wants to marry you, or your own mother turned your stepsister's head into a calf's head. But other times you set out to seek your fortune. Except that typically you did so to leave your parents' home and to find your own spouse and settle down in a new household where you're the parent. So adolescence in a sense is forcing you out.
YA. This is because if an adult goes out on a quest, one has the question of on one hand, what is happening with the wife and children and employer, or any rate the people relying on his work, or on the other hand, did this guy really get to this age without acquiring any responsibilities, and if so, is he really responsible enough for the quest? You can finesse it, killing all them so he's out for revenge is classic, but you don't have to finesse it if the hero's adolescent.