Under-age rhymes with:
age, assuage, backstage, bethpage, cage, disengage, drage, engage, enrage, gage, gaige, gauge, grage, hage, kage, krage, lage, offstage, onstage, osage, page, paige, rage, restage, sage, schrage, stage, upstage, wage, wrage
Under-age sounds like:
underage, undercook, undergo, undergoes, undersea, underseas, underuse, undress