Source : Webster's Revised Unabridged Dictionary (1913)

Under-age \Un"der-age`\, a.
   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.