un·der·age /ˌʌndɚ/
  (a.)未成年的;未達成年的
  un·der·age a. Shortage or deficiency in amount; shortfall. [Obs.]
  2. Not having arrived at adult age, or at years of discretion; hence, raw; green; immature; boyish; childish. [Obs.]
     I myself have loved a lady, and pursued her with a great deal of under-age protestation.   --J. Webster.
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  underage
       adj 1: not of legal age; "minor children" [syn: minor, nonaged]
              [ant: major]
       2: dependent by virtue of youth