In the d20 system I use we have these adjusments:
Child (1–11): –2 to Str and Con; +2 to Dex; –1 Int, Wis, and Cha
Young adult (12–15): –1 to Str and Con; –1 Int and Wis
Adult (16–39): Base scores
General idea is that kids aren't as smart or as tough and strong as adults but more dextrous, and preteens/early teens, while stronger and tougher than children, still aren't as much so as an adult, and still not fully mentally developed.