The Collaborative International Dictionary of English v.0.48:

Young \Young\ (y[u^]ng), a. [Compar. Younger
   (y[u^][ng]"g[~e]r); superl. Youngest (-g[e^]st).] [OE.
   yung, yong, [yogh]ong, [yogh]ung, AS. geong; akin to OFries.
   iung, iong, D. joing, OS., OHG., & G. jung, Icel. ungr, Sw. &
   Dan. ung, Goth. juggs, Lith. jaunas, Russ. iunuii, L.
   juvencus, juvenis, Skr. juva[,c]a, juvan. [root]281. Cf.
   Junior, Juniper, Juvenile, Younker, Youth.]
   [1913 Webster]
   1. Not long born; still in the first part of life; not yet
      arrived at adolescence, maturity, or age; not old;
      juvenile; -- said of animals; as, a young child; a young
      man; a young fawn.
      [1913 Webster]

            For he so young and tender was of age. --Chaucer.
      [1913 Webster]

            "Whom the gods love, die young," has been too long
            carelessly said; . . . whom the gods love, live
            young forever.                        --Mrs. H. H.
                                                  Jackson.
      [1913 Webster]

   2. Being in the first part, pr period, of growth; as, a young
      plant; a young tree.
      [1913 Webster]

            While the fears of the people were young. --De Foe.
      [1913 Webster]

   3. Having little experience; inexperienced; unpracticed;
      ignorant; weak.
      [1913 Webster]

            Come, come, elder brother, you are too young in
            this.                                 --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Younger \Young"er\, n.
   One who is younger; an inferior in age; a junior. "The elder
   shall serve the younger." --Rom. ix. 12.
   [1913 Webster]

   2. Used of the younger of two persons of the same name
      especially used to distinguish a son from his father; --
      usually used postpositionally; as, Henry the younger.

   Syn: jr.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

younger
    adj 1: used of the younger of two persons of the same name
           especially used to distinguish a son from his father;
           "John Junior"; "John Smith, Jr." [syn: younger, jr.]