成人年龄是属于在法律内，一种为了与成年人与未成年人之间分清楚界限的其中一种定义。这是从按时间顺序排列来说，当未成年人到达了不再具有法律条文被认定为是儿童，并承担了他们的人、行动和决策实施控制，从而终止父母或监护人监护未成年人而受法律控制的责任。有很多国家把法定年龄都定义在18岁的年龄阶段。其中，“法定年龄”的“法定”二字指的是满足或超过该周岁的年龄，与未成年人是一个相对的概念。但在某些国家内的部分法律，可能不会使用“法定年龄”此术语来制定这样赐予成年人与未成年人年龄地段的法律地位。其中，法定年龄是固定的观念和准则，然而这一个观念不一定代表一个人的真实心理会导致成熟。虽然一个人到达合法年龄时，能享受任何权利或义务，但这些权利或义务也受到一定程度的法律限制（不是无政府状态）。例如参选什幺高等职位的权利，都是有设定年龄的限制。

The age of majority is the threshold of adulthood as it is conceptualized (and recognized or declared) in law. It is the chronological moment when minors cease to legally be considered children and assume control over their persons, actions, and decisions, thereby terminating the legal control and legal responsibilities of their parents or guardian over and for them. Most countries set majority at 18. The word majority here refers to having greater years and being of full age; it is opposed to minority, the state of being a minor. The law in a given jurisdiction may never actually use the term "age of majority" and the term thereby refers to a collection of laws bestowing the status of adulthood. The age of majority is a legally fixed age, concept, or statutory principle, which may differ depending on the jurisdiction, and may not necessarily correspond to actual mental or physical maturity of an individual.