youthful

adjective

1. characterized by youth; young.
2. of, relating 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.

adjective

1. of, relating to, possessing, or characteristic of youth
2. fresh, vigorous, or activehe’s surprisingly youthful for his age
3. in an early stage of developmenta youthful culture
4. Also: young (of a river, valley, or land surface) in the early stage of the cycle of erosion, characterized by steep slopes, lack of flood plains, and V-shaped valleysCompare mature (def. 6), old (def. 18)

adj.1560s, from youth + -ful. Related: Youthfulness.