Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   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. -- {Youth"ful*ly}, adv. -- {Youth"ful*ness},
          n.

Source : WordNet®

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