The Collaborative International Dictionary of English v.0.48:

Lawful \Law"ful\, a.
   1. Conformable to law; allowed by law; legitimate; competent.
      [1913 Webster]

   2. Constituted or authorized by law; rightful; as, the lawful
      owner of lands.
      [1913 Webster]

   Lawful age, the age when the law recognizes one's right of
      independent action; majority; -- generally the age of
      twenty-one years. Also called legal age or age of
      majority.
      [1913 Webster +PJC]

   Note: In some of the States, and for some purposes, a woman
         attains lawful age at eighteen. --Abbott.

   Syn: Legal; constitutional; allowable; regular; rightful.

   Usage: Lawful, Legal. Lawful means conformable to the
          principle, spirit, or essence of the law, and is
          applicable to moral as well as juridical law. Legal
          means conformable to the letter or rules of the law as
          it is administered in the courts; conformable to
          juridical law. Legal is often used as antithetical to
          equitable, but lawful is seldom used in that sense. --
          Law"ful*ly, adv. -- Law"ful*ness, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

legal age
    n 1: the age at which persons are considered competent to manage
         their own affairs [syn: majority, legal age] [ant:
         minority, nonage]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "legal age":
   adulthood, adultness, age of consent, driving age, flower of age,
   full age, full bloom, full growth, fullgrownness, grown-upness,
   legalis homo, majority, manhood, manlihood, mature age, maturity,
   prime, prime of life, ripe age, riper years, toga virilis,
   virility, womanhood, womanlihood, years of discretion