Why are so many heroes of fantasy (adult fantasy and YA fantasy alike) young?
I skip lightly over the marketing reason (to appeal to the target demographic) partly because I find the aesthetic reasons more interesting, and partly because it's much older than that. Some fairy tales start out with children, young enough to return to the parental home after adventure is done; still fewer start out with an adult who left the parental home (and possibly even married and founded his own home) going on adventures. But the main plotline is generally the hero is living in his parents' house, leaves (voluntarily or involuntarily), has adventures, marries and settles down in a new home.
This has several advantages in fantasy.
If you want to send the hero on a quest, who is he leaving behind in a lurch? Spouse, children, job? A little less than heroic there. No one? But if it's no one, how did an older character manage to get to his current age without acquiring responsibilities? A little self-centered, there, mostly likely. A sixteen year old, on the other hand, can have no responsibilities without any more reason than age. And in the reverse manner, we have fewer questions about how his parents (foster parents, guardians, what have you) let him go.
It gives more room for growth. The character can grow, mature, develop -- and not raise so many questions about how he started out so immature at his age. Developing abilities and learning about the past are less time bound, but if the character is older, it may raise unsettling questions about his lack of curiosity.
Romance subplots. The absence of a spouse and children can be crucial here. And the question of why the hero is still unaffiliated at his age is even more pointed. Is there something seriously wrong with this person, such that no one would want a romance with him?
And, on the symbolic side, characters at these ages are liminal. Neither adult nor children. Goes hand in hand with all the other liminal traits they often acquire -- going on journeys, to be in neither one place nor another; transforming into a wizard, or a swordsman, or a king -- and reinforces them.
Doesn't apply to all genres. Ancient myths and legends frequently had fully adult heroes, and so does sword and sorcery, drawing on it. OTOH, the heroes in sword and sorcery tend to all have the common character trait of loving danger or change. Settling in one place is dull, dull, dull, and often only danger attracts them. Romantically, they are notoriously love-'em-and-leave-'em types. Which finesses all the issues except liminality, but which boxes the writer in in other ways.
Urban fantasy is more flexible, because modern culture is more flexible. Not marrying young may still need some explanation, but nothing so much as if the character was a noble in a culture with arranged marriages. By the same token, it boxes you in on your world-building.
On the other hand, even in genres where all these advantages apply in full, they can be finessed. Massacring the hero's village is such a good way of disencumbering him of all responsibilities (and leaving him open to new romance) that it's a cliche, and even used on the younger heroes -- partly, of course, because of its other advantage, namely powerfully motivating the hero. Threatening or kidnapping the family can also be used in the same manner. Conversely, making him unable to marry or take a significant job can give a motive to escape the situation, particularly if the reason is that people were treating him badly. Or even a character who has lost responsibility: the oldest of a family of orphans, when the youngest orphan has just married, or entered a guild, or somehow ceased to be a responsibility.
Still, even if you start with the notion of an older character, you have to somehow make the finessing work in the story. It's not enough to throw it in. It has to be integral, and advance the story. If not, a younger character would be better.