最佳解答: Young, youthful, juvenile all refer to lack of age.
(1) Young is the general word for that which is undeveloped, immature, and in process of growth:
Young 是未開展的，未成熟的, 還在成長的過程中的一般詞語
A young colt, child; young shoots of wheat.
(2) Youthful has connotations suggesting the favorable characteristics of youth, such as vigor, enthusiasm, and hopefulness:
年輕有內涵 (指青年時期的有利特徵) - 例如強健、熱情和充滿希望的人：
Youthful sports, energy, outlook.
(3) If you are interested,
Juvenile may suggest less desirable characteristics, such as childishness, petulance, idleness, selfishness, or heedlessness
or it may refer simply to the years, up to the later teens, before legal responsibility:
juvenile delinquency; juvenile court; juvenile books.
少年犯罪; 少年法庭; 青少年讀的書