For civil purposes, "of legal age" is 18. (Except alcohol usage).

Contracts signed my minors are voidable. This means that once they reach 18 they have the option of voiding their contract. If they don't after a "resonable time", then they "affirm" the contract. That's why you should never do business with minors! :-) Most states have special clauses for this situation. Texas protects businesses as well as minors.