To be fair, I've heard that young adult crap back when I was still a minor in high school. It kind of felt like a matured version of "you're a big boy/girl now~" but in reality they still see you as a brat. So~ this confirms nothing to me, except that my character is at minimum 14 years old. Cause nobody brings out the young adult line for a 12 year old. ¯\_(ツ)_/¯
Though... it's fiction and your imagination, so sure, 12 year old get the young adult line now, lol.
I think the smallest size used to be eight. Which, not gonna lie, touch weird, but whatevs.