The Collaborative International Dictionary of English v.0.48:

underage \un"der*age`\ ([u^]n"d[~e]r*[i^]j), a.
   Shortage or deficiency in amount; shortfall. [Obs.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Under-age \Un"der-age`\, a.
   1. Not having arrived at adult age; hence, incapable legally
      of performing certain acts restricted to adults.
      [PJC]

   2. Not having arrived at adult age, or at years of
      discretion; hence, raw; green; immature; boyish; childish.
      [Obs.]
      [1913 Webster]

            I myself have loved a lady, and pursued her with a
            great deal of under-age protestation. --J. Webster.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dependent \De*pend"ent\, a. [L. dependens, -entis, p. pr.
   dependere. See Depend, and cf. Dependant.]
   1. Hanging down; as, a dependent bough or leaf.
      [1913 Webster]

   2. Relying on, or subject to, something else for support; not
      able to exist, or sustain itself, or to perform anything,
      without the will, power, or aid of something else; not
      self-sustaining; subordinate; -- often with on or upon;
      as, dependent on God; dependent upon friends. Opposite of
      independent. [Narrower terms: interdependent,
      mutualist, mutually beneficial; parasitic, parasitical,
      leechlike, bloodsucking; subordinate; underage;
      myrmecophilous; symbiotic] Also See: unfree.
      [1913 Webster]

            England, long dependent and degraded, was again a
            power of the first rank.              --Macaulay.
      [1913 Webster]

   3. conditional; contingent or conditioned. Opposite of
      unconditional.

   Syn: qualified.
        [WordNet 1.5]

   4. addicted to drugs.

   Syn: addicted, dependent, drug-addicted, hooked, strung-out.
        [WordNet 1.5]

   Dependent covenant or Dependent contract (Law), one not
      binding until some connecting stipulation is performed.

   Dependent variable (Math.), a varying quantity whose
      changes are arbitrary, but are regarded as produced by
      changes in another variable, which is called the
      independent variable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

underage
    adj 1: not of legal age; "minor children" [syn: minor,
           nonaged, underage] [ant: major]
    2: dependent by virtue of youth

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "underage":
   budding, callow, defalcation, deficiency, deficit, dewy, green,
   growing, immature, impubic, inadequacy, inexperienced, ingenuous,
   innocent, insufficiency, intact, juicy, lack, minor, naive,
   new-fledged, raw, ripening, sappy, scantiness, shortage, tender,
   unadult, undeveloped, unfledged, unformed, unlicked, unmellowed,
   unripe, unseasoned, vernal, virginal