Most of the time I feel like YA has to be careful with the form and pacing of a story so a prologue can sometimes be risky. Adult fantasy readers are used to prologues and slow introductions/pacing for long stories, but I think YA fantasy usually tries to capture the younger readers' attention fairly quickly (YA fantasy, especially the first book in a series, is much shorter for the same reason).
If the prologue is important enough to the story it'll probably work well as a first chapter. I'm thinking of The Boy Who Lived - chapter one of the Harry Potter series - it functions as a prologue, but presenting it as the first chapter just helps the story flow and helps kids get into the story quickly. That's Middle Grade, though, and there's a little more leeway in YA to experiment with form. When I have seen prologues they've been really just a short scene, a few hundred words, definitely nothing as long as you see in adult fantasy. Still, I think it's important to pay attention to genre conventions when writing for younger readers. YA fantasy is having a surge in popularity, but not every fantasy story is YA even if the characters are younger.
Speaking of genre conventions...I really do wish booksellers and marketing was more upfront about Sarah J. Maas' move into adult fantasy. It really bothers me that her ACOTAR series and the later TOG books are still sold and marketed as YA fantasy. I know they're just trying to make use of the loyal audience, but sometimes I think the YA community needs to remember that as a genre it's meant to be for actual teens, no matter how many adults enjoy YA books. Not that I'm against sex in YA - I think it has a really important place in the genre, especially in contemporary non-fantasy YA, and can give teens a great space for considering the emotional side of intimacy and its role in personal development - but Maas' sex scenes are a lot more like bodice-ripping romance scenes and I don't think that kind of sex scene is very useful for the YA genre and actual teen readers. Especially given how many middle grade readers start reading up specifically within the fantasy genre.
Sorry I have a lot of thoughts on both of these issues...well just issues about the YA genre in generally, especially anything that moves the genre away from its purpose for fostering actual teen reading experiences.