What term describes someone who is young or immature?

The term that describes someone who is young or immature is "juvenile." This word is often used in legal contexts to refer to individuals who are not yet adults, typically those under the age of 18. It encompasses anyone in the developmental stages of life, highlighting their youth and the associated characteristics of being less developed in various capacities, such as emotionally, socially, or physically.

In contrast, the other terms refer to people at different life stages. An "adult" denotes someone who has reached full physical and emotional maturity, while "senior" typically refers to older adults, often those who are retired or approaching retirement age. The term "mature" suggests an advanced state of development, both emotionally and intellectually. These terms indicate stages of life that are not associated with youth or immaturity, making "juvenile" the most accurate choice in this context.