having qualities of youth; appearing young, energetic, or having the characteristics of young people.
From Old English 'geogeth' (youth) plus suffix '-ful' (full of). 'Youth' itself comes from Germanic roots meaning 'joined' or 'vigorous.'
Youthful is interesting because it's the only positive way we have to describe acting young—'juvenile' sounds silly, 'childish' sounds immature, but 'youthful' is always flattering, showing how language protects dignity.
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