Webster's Revised Unabridged Dictionary · 1913

Youthful

Youthful , adjective

1.
Not yet mature or aged; young.
Also used figuratively.
Two youthful knights. — Dryden
The youthful season of the year. — Shakespeare
2.
Of or pertaining to the early part of life; suitable to early life; as, youthful days; youthful sports.
Warm, youthful blood. — Shakespeare
Youthful thoughts. — Milton
3.
Fresh; vigorous, as in youth.
After millions of millions of ages... still youthful and flourishing. — Bentley