There Is A Difference Between Middle Grade and Young Adult Lit, and It Does Matter 

Adolescence took a long time to be invented at all, and it took an even longer time to become a somewhat stable archetype. It’s only in recent human history that people a) live long enough for adolescence to be a different physical experience than adulthood; and b) live in a society that indulges an extended period of fooling around, experimenting, and generally doing nothing to benefit society. And it’s really only in the last few decades that medical advances have allowed us to understand that adolescent brains truly are distinct from children’s and adults’ brains, and they respond to stimuli in a different way

Source: There Is A Difference Between Middle Grade and Young Adult Lit, and It Does Matter | The Learned Fangirl