precocious

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Manifesting or characterized by development, aptitude, or interests considered advanced for a given age.
• adjective Botany Blossoming before the appearance of leaves.

from The Century Dictionary.

• In biology, present in an egg or embryo at a very early stage in an imperceptible condition; present before becoming manifest.
• Noting birds that, like chickens, are able to run about as soon as hatched; precocial. See Præcoces.
• Ripe before the natural time.
• Ripe in understanding at an early period; prematurely developed; forward: as, a precocious child; precocious faculties.
• Indicative of precocity; characteristic of early maturity; anticipative of greater age; premature.
• In botany, appearing before the leaves: said of flowers.

from the GNU version of the Collaborative International Dictionary of English.

• adjective rare Ripe or mature before the proper or natural time; early or prematurely ripe or developed.
• adjective 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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Characterized by exceptionally early development or maturity.
• adjective Exhibiting advanced skills at an abnormally early age.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective characterized by or characteristic of exceptionally early development or maturity (especially in mental aptitude)
• adjective appearing or developing early