Child Soldier (3.5e Trait)
From D&D Wiki
A warrior at an early age, their maturity doesn't match their age, but no one ever takes them seriously... until its too late. The Character must be under the starting age for their race. Characters with this trait over the age or under the affects of age altering spells, lose the trait.
Benefit: +2 to all Cha and Dex skill checks except Intimidate. Allows characters who are not adults to be able to move, act and cast as if they were adults. Characters that take this feat are treated as one size category smaller in size until they grow to the minimum starting age for their race.
Drawback: -2 to all Str and Wis skill checks as well as -4 to Intimidate. Adults will not take your character seriously until you've proven you are a forced to be reckoned with.