It’s important to understand that YA is not a genre – it’s simply the group you are marketing to. There really is no difference between “mature” and “immature” YA. It’s either Young Adult, or it’s not. There seems to be some discrepancy about the age groups that should be targeted, with some saying YA starts at age 12, and others saying they should be older. In reality, the age group is simply a guideline and you may have mature adults who enjoy YA novels as much as a sixteen-year-old.
It’s not your job to determine who, outside of the generalized age group, will enjoy your book. Stick to your market.
So, how do you determine if you’re writing YA?
The first clue is your characters. You want them to relate to your readers as much as possible and, since your market is mainly teenagers, your characters should be teenagers or very young adults, at least.
Next, look at how the story is written. What voice are you writing it in? Does it seem complex and full of wording and phrases that might require some research for the reader to understand? If so, then you are targeting an older age group and should reclassify your book.