youth·ful /ˈyuθfəl/ Show Spelled[yooth-fuhl] Show IPA –adjective 1. characterized by youth; young. 2. of, pertaining to, or befitting youth: youthful enthusiasm. 3. having the appearance, freshness, vigor, etc., of youth: She is 60 but her optimism has kept her youthful. 4. in an early period of existence; early in time. 5. Physical Geography . (of topographical features) having undergone erosion to a slight extent only.