Or it is the difference between the first person and the third person? Just finished Canticle by Ken Scholes which is third person adult and have now started reading Forest of Hands and Teeth by Carrie Ryan, first person young adult. Two of the six or eight main characters of Canticle are young adults, and the rest of adult adults. Those parts that are about the two young adult characters -- Neb and Winters -- would be very much at home in a young adult novel. In fact, I often wished the novel were just about them. The third-person limited POV was not so far from first person, though it allows for a more expansive, elaborate narrative voice than would have been the case with a first person narrative. Carrie Ryan's narrator (Mary) tells her story in much more simple, though still subtly complex, poetic language. There seems to be no particular reason why the first person young adult narrator cannot feature in an adult book: I'm sure I could find many examples. All of my big three of YA fantasy novels -- LOTR, His Dark Materials, and HP -- have a subjective third person pov, though Pulliam stays pretty close to his two young protagonists. Mrs. Coulter's behaviors and actions are seen only through Lyra. LOTR is not a young adult novel at all, it's just a favorite of many young adults. Does that make it a young adult novel?
For my own little project, I've chose alternating first person, though I may use a subjective third person in the last volume. The language of my characters must be different, and I can see no reason why these characters can't make careful observations or use language in inventive ways. Who would want to read it if they didn't?