from the d20 Modern SRD;
Children
Children (newborns to age 11) are handled differently from other characters. They do not have classes or levels. They begin with the same ability score package as ordinaries (15, 14, 13, 12, 10, 8), but their ability scores are reduced as follows: -3 Str, -1 Dex, -3 Con, -1 Int, -1 Wis, -1 Cha.
Children have 1d4 hit points plus their Constitution modifier (minimum 1 hit point). They have no skills, feats, action points, or occupations. Their base attack bonus is +0, they have a +0 modifier on all saving throws (plus any modifiers for high or low ability scores), and their Reputation bonus is +0. Children have a +0 modifier to Defense and a normal speed of 20 feet. Children have no effective attacks and should be treated as noncombatants.
When a child turns 12, he or she is considered a young adult and takes his or her first level in one of the six basic classes. At that point, the character becomes an ordinary (or hero, in some cases).