In the d20 system I use we have these adjusments:

Child (111): 2 to Str and Con; +2 to Dex; 1 Int, Wis, and Cha
Young adult (1215): 1 to Str and Con; 1 Int and Wis
Adult (1639): 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.