The Collaborative International Dictionary of English v.0.48:

Youth \Youth\ ([=u]th), n.; pl. Youths ([=u]ths; 264) or
   collectively Youth. [OE. youthe, youh[thorn]e,
   [yogh]uhe[eth]e, [yogh]uwe[eth]e, [yogh]eo[yogh]e[eth]e, AS.
   geogu[eth], geogo[eth]; akin to OS. jugu[eth], D. jeugd, OHG.
   jugund, G. jugend, Goth. junda. [root]281. See Young.]
   [1913 Webster]
   1. The quality or state of being young; youthfulness;
      juvenility. "In my flower of youth." --Milton.
      [1913 Webster]

            Such as in his face
            Youth smiled celestial.               --Milton.
      [1913 Webster]

   2. The part of life that succeeds to childhood; the period of
      existence preceding maturity or age; the whole early part
      of life, from childhood, or, sometimes, from infancy, to
      manhood.
      [1913 Webster]

            He wondered that your lordship
            Would suffer him to spend his youth at home. --Shak.
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            Those who pass their youth in vice are justly
            condemned to spend their age in folly. --Rambler.
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   3. A young person; especially, a young man.
      [1913 Webster]

            Seven youths from Athens yearly sent. --Dryden.
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   4. Young persons, collectively.
      [1913 Webster]

            It is fit to read the best authors to youth first.
                                                  --B. Jonson.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

youth
    n 1: a young person (especially a young man or boy) [syn: young
         person, youth, younker, spring chicken]
    2: young people collectively; "rock music appeals to the young";
       "youth everywhere rises in revolt" [syn: young, youth]
       [ant: aged, elderly]
    3: the time of life between childhood and maturity
    4: early maturity; the state of being young or immature or
       inexperienced
    5: an early period of development; "during the youth of the
       project" [syn: youth, early days]
    6: the freshness and vitality characteristic of a young person
       [syn: youth, youthfulness, juvenility]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "youth":
   adolescence, adolescent, adolescents, awkward age, baby, babyhood,
   beginnings, birth, boy, boyhood, bub, bubba, buck, bud, buddy,
   callowness, chick, child, childhood, childkind, children, chit,
   colt, cradle, cub, damsel, demoiselle, dewiness, fellow, fledgling,
   freshman year, genesis, girl, girlhood, greenness, hobbledehoy,
   hopeful, immaturity, inception, inchoation, incipience, incipiency,
   incunabula, inexperience, infancy, infant, junior, juvenal,
   juvenile, juveniles, juvenility, kid, kids, lad, laddie, lass,
   lassie, little kids, mademoiselle, maid, maiden, manchild, master,
   minor, minority, moppet, muchacho, nascence, nascency, nativity,
   new generation, origin, origination, parturition, pregnancy, prime,
   puberty, pubescence, pubescent, pup, puppy, rising generation,
   salad days, sapling, schoolboy, schoolgirl, slip, small fry, sonny,
   sonny boy, sprig, spring, springtide, springtime, stripling, tad,
   teen, teenager, teener, teenybopper, tots, unripeness, whelp,
   whippersnapper, young, young blood, young fry, young hopeful,
   young man, young people, young person, younger, youngest,
   youngling, youngster, youngsters, youthfulness, youthhood



The Devil's Dictionary (1881-1906):

YOUTH, n.  The Period of Possibility, when Archimedes finds a fulcrum,
Cassandra has a following and seven cities compete for the honor of
endowing a living Homer.

        Youth is the true Saturnian Reign, the Golden Age on earth
    again, when figs are grown on thistles, and pigs betailed with
    whistles and, wearing silken bristles, live ever in clover, and
    clows fly over, delivering milk at every door, and Justice never
    is heard to snore, and every assassin is made a ghost and,
    howling, is cast into Baltimost!
                                                        Polydore Smith