Young people don't want everything to be YA, they want to be taken seriously and be served serious entertainment fare as well.
I agree. Sure, there was no shortage of YA material when I was a kid ... the Hardy Boys, John Christopher's Tripods books, the Chronicles of Narnia, and even The Hobbit come to mind. But I also liked The Lord of the Rings, Asimov, and plenty of other adult fiction besides. I didn't need Luke Skywalker to be a six-year-old to relate to him when I was six myself.
But the thing is, I bet it's just more "audiencing" by the studio execs. They want all movies to appeal to the broadest possible audience. And what do all potential audience members have in common, whether they be make, female, LGBTQ, Asian, European, Black, etc.? They were all young once.