What age do guys start to mature physically and mentally?

Hi currently I'm in Grade 12 and it seems as though half the guys in my grade haven't matured in (height, voice tone or mentally mature) yet. How come is it that the average age states that they mature both physically and mentally by the age of 15. Thats impossible. I'm taller than half the guys in my grade, though I'm deemed as average as a 17 year old girl. I'm 5'' 7 and most of the guys are up to my chin, is this normal. Another thing is that grade 12 boys are far more immature than the grade 8 boys in which I've tutored for grad credits. One guy in my biology cl today was talking about having with the substitute, that which was subbing today for our teacher. Please can someone tell me why they act soooo immature and they're in grade 12 already like mini adults. I thought that in grade 12 they would all change and turn into adults ughh!!!!!!! I hate it when I went to the opening dance and I was dancing with a midget. Ok thats not a bad thing but......uhhhh...yeah:P