Naruto is said to have failed the graduation exams three times because bunshin was required and it's his worst skill before he finally learned kage bunshin. Yet geniuses like Sasuke and Neji graduated at the same age.

Zabuza was reported to have been a young boy when he killed a hundred shinobi students ten years ago. He would have been 16 at the time, hardly a boy.

Itachi is said to have made chuunin at age 10 and joined the ANBU only 6 months after being a chuunin. He was supposed to be a Anbu captain at 13, but it seems he wasn't with the ANBU that long and he didn't look 10 when he was shown joining the ANBU.