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A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles can look very different from the adult form, particularly in colour. In many organisms the juvenile has a different name from the adult (see List of animal names).
Some organisms reach sexual maturity in a short metamorphosis, such as eclosion in many insects. For others, the transition from juvenile to fully mature is a more prolonged process—puberty in humans, for example. In such cases, juveniles during this transformation are sometimes called subadults.
In vertebrates and some invertebrates (e.g. spiders), larval forms (e.g. tadpoles) are usually considered a development stage of their own, and "juvenile" refers to a post-larval stage that is not fully grown and not sexually mature. In amniotes and most plants, the embryo represents the larval stage. Here, a "juvenile" is an individual in the time between hatching/birth/germination and reaching maturity.
- For animal larval juveniles, see larva
- For human juvenile life stages, see childhood and adolescence, an intermediary period between the onset of puberty and full physical, psychological, and social adulthood