What term is used to describe someone or something that is not fully grown or immature?

The term "juvenile" specifically refers to an individual or entity that is not fully grown or is still in an immature stage of development. This term is commonly used in various contexts, such as biology, where it describes young animals, or in legal contexts to refer to minors or young offenders. In contrast, the other terms provided either denote a mature stage of life or represent specific age groups. "Adult" refers to a fully developed person or organism, "neonate" specifically describes a newborn, typically in the first few weeks of life, and "senior" refers to older adults, often implying advanced age or maturity. Thus, "juvenile" accurately captures the essence of immaturity or being not fully grown.