A more thorough explanation:

Definition: A period of time, especially a period of individual existence or the duration of a person's life. In American usage, age is stated in full years completed.

Examples:

• Age of capacity: The age, usually defined by statute as 18 years, at which a person is legally capable of agreeing to a contract, maintaining a lawsuit, or the like. For example, a person can sign a lease agreement or file a lawsuit once they reach the age of capacity.
• Age of consent: The age, usually defined by statute as 16 years, at which a person is legally capable of agreeing to marriage (without parental consent) or to sexual intercourse. For example, if a person over the age of consent has sexual intercourse with a person under the age of consent, the older person may be prosecuted for statutory rape regardless of whether the younger person consented to the act.
• Age of criminal responsibility: The age at which a child may be held responsible for a criminal act. For example, in some states, a child as young as 7 can be held responsible (as a juvenile) for some acts.
• Age of majority: The age, usually defined by statute as 18 years, at which a person attains full legal rights, especially civil and political rights such as the right to vote. For example, a person can vote in elections once they reach the age of majority.

These examples illustrate how age is defined and used in different legal contexts. The age of capacity determines when a person can make certain legal decisions, the age of consent determines when a person can engage in sexual activity, the age of criminal responsibility determines when a child can be held accountable for their actions, and the age of majority determines when a person attains full legal rights.