The Collaborative International Dictionary of English v.0.48:

Precocious \Pre*co"cious\, a. [L. praecox, -ocis, and
   praecoquus, fr. praecoquere to cook or ripen beforehand; prae
   before + coquere to cook. See 3d Cook, and cf. Apricot.]
   1. Ripe or mature before the proper or natural time; early or
      prematurely ripe or developed; as, precocious trees. [R.]
      --Sir T. Browne.
      [1913 Webster]

   2. Developed more than is natural or usual at a given age;
      exceeding what is to be expected of one's years; too
      forward; -- used especially of mental forwardness; as, a
      precocious child; precocious talents.
      [1913 Webster]

WordNet (r) 3.0 (2006):

precocious
    adj 1: characterized by or characteristic of exceptionally early
           development or maturity (especially in mental aptitude);
           "a precocious child"; "a precocious achievement" [ant:
           retarded]
    2: appearing or developing early; "precocious flowers appear
       before the leaves as in some species of magnolias"

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "precocious":
   a bit previous, advanced, ahead, bright, developed, early,
   far ahead, forward, gifted, half-baked, half-cocked, hasty,
   ill-considered, impulsive, intelligent, mature, not firm,
   overhasty, oversoon, precipitate, premature, previous, quick,
   rushed, smart, soon, too early, too soon, uncrystallized, unjelled,
   unmatured, unmeditated, unpremeditated, unprepared, unripe,
   untimely