Youthful, a.
1. Not yet mature or aged; young. "Two youthful knights." Dryden.
Also used figuratively. "The youthful season of the year." Shak.
2. Of or pertaining to the early part of life; suitable to early life; as, youthful days; youthful sports. "Warm, youthful blood." Shak. "Youthful thoughts." Milton.
3. Fresh; vigorous, as in youth. After millions of millions of ages . . . still youthful and flourishing. Bentley.
1 more senses
Syn. -- Puerile; juvenile. -- Youthful, Puerile, Juvenile. Puerile is always used in a bad sense, or at least in the sense of what is suitable to a boy only; as, puerile objections, puerile amusements, etc. Juvenile is sometimes taken in a bad sense, as when speaking of youth in contrast with manhood; as, juvenile tricks; a juvenile performance. Youthful is commonly employed in a good sense; as, youthful aspirations; or at least by way of extenuating; as, youthful indiscretions. "Some men, imagining themselves possessed with a divine fury, often fall into toys and trifles, which are only puerilities." Dryden. "Raw, juvenile writers imagine that, by pouring forth figures often, they render their compositions warm and animated." Blair. -- Youth"ful*ly, adv. -- Youth"ful*ness, n.
Shades of meaning
Boyish, childish, and girlish are used in a good sense of those to whom they properly belong, but in a bad sense of those from whom more maturity is to be expected; childish eagerness or glee is pleasing in a child, but unbecoming in a man; puerile in modern use is distinctly contemptuous. Juvenile and youthful are commonly used in a favorable and kindly sense in their application to those still young; youthful in the sense of having the characteristics of youth, hence fresh, vigorous, light-hearted, buoyant, may have a favorable import as applied to any age, as when we say the old man still retains his youthful ardor, vigor, or hopefulness; juvenile in such use would belittle the statement. Young is distinctively applied to those in the early stage of life or not arrived at maturity. Compare new.