Responses
I think it’s also the face as well. In other yaoi faces similar to his usually belong to middle schoolers/ younger kids/teens. The large eyes probably add to it as well. Ukes tend to have bigger/ rounder eyes and semes smaller/more rectangular(?)/ masculine(?) eyes, but here the seme’s eyes are already the bigger/rounder type, so making the uke’s even bigger/rounder makes him look younger.

Is it me or are the size proportions kind of off between the two of them? Like. Hibari looks a lot younger than he is because his head is like half the size. I can't seem him as anything past middle school cause of it. It's ok if he's short, but his head is tiny in comparison.