youthful

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

• adjective Characterized by youth; young.
• adjective Typical of or suggesting youth: synonym: young.
• adjective In an early stage of development; new.
• adjective Geology Young.

from The Century Dictionary.

• In physical geography, exhibiting an early stage of the geographical cycle; young.
• Possessing or characterized by youth; not yet aged; not yet arrived at mature years; being in the early stage of life; young; juvenile.
• Pertaining or belonging or suitable to the early part of life: as, youthful days; youthful age.
• Fresh and vigorous, as in youth.
• Early in time.
• =Syn.1-3. Youthful, Juvenile, Boyish, Puerile. Youthful is generally used in a good sense: as, youthful looks or sports; juvenile indifferently, but if in a bad sense not strongly so: as, the poem was a rather juvenile performance; boyish rather more often, but not necessarily, in some contempt: as, a boyish manner; boyish enthusiasm; puerile always in marked contempt, as a synonym for silly.

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

• adjective Not yet mature or aged; young.
• adjective Of or pertaining to the early part of life; suitable to early life
• adjective Fresh; vigorous, as in youth.

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

• adjective Young or seeming young.
• adjective Characteristic of young people.

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

• adjective suggestive of youth; vigorous and fresh