Youthful means having the freshness of the young. A youthful appearance, youthful enthusiasm, youthful indiscretion.
Applied to somebody older it is a compliment — a youthful seventy — and applied to a mistake it is an excuse: a youthful indiscretion is one forgiven on account of age. Youthful energy and youthful optimism are the standard pairs. Note the difference from young, which is a fact, and from childish and juvenile, which are criticisms. Youthful is the only one of the four that is reliably kind.
1adjectiveyouthfulHaving the freshness of the young.
“He was youthful at about seventy and walked nine miles.”
synonyms: young-looking, fresh
youthful is a quality of youth.
Youth plus -ful. Youth is from Old English geoguth, related to young, and the -ful ending means full of rather than the modern almost-suffix that only marks an adjective.
Proto-Germanic
*jugunþō
Proto-West Germanic
*juwunþa
Old English
ġeoguþ — the state of being young; youth
Middle English
youthe
English
youth
English, today
youthful
Built from
From youth + -ful. youth comes from Middle English youthe, youghte, ȝouþe, from Old English ġeoguþ (“the state of being young; youth”), from Proto-West Germanic juwunþa, from Proto-Germanic jugunþō, jugunþiz (“youth”), corresponding to young + -th. Cognate with stq Juugd, West Frisian jeugd, Dutch jeugd, German Low German Jöögd, German Jugend.
Etymology from Wiktionary, CC BY-SA