Every writers that is serious about publishing, particularly those attempting the traditional publication path, will know that agents and editors put a lot of weight on word counts. The acceptable limits vary depending on the age category and genre of the book.
(By the way, Young Adult is NOT a genre. It’s an age category. And Fiction is NOT a genre either. You can find more information about the various age categories here. More information about the main genre classifications can be found here.)
It’s incredibly important to have a good understanding of the average word counts for the type of story that you are writing, but it’s just as important to understand word counts are not an excuse for poor story telling.