The Collaborative International Dictionary of English v.0.48:

Youthful \Youth"ful\, a.
   1. Not yet mature or aged; young. "Two youthful knights."
      --Dryden. Also used figuratively. "The youthful season of
      the year." --Shak.
      [1913 Webster]

   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.
      [1913 Webster]

   3. Fresh; vigorous, as in youth.
      [1913 Webster]

            After millions of millions of ages . . . still
            youthful and flourishing.             --Bentley.
      [1913 Webster]

   Syn: Puerile; juvenile.

   Usage: 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.
          [1913 Webster] -- Youth"ful*ly, adv. --
          Youth"ful*ness, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

youthfully
    adv 1: in a youthful manner; "he is still youthfully
           enthusiastic"