noun
legal age
Refers to the age at which a person is legally permitted to do something, such as drink, vote, or marry.
日本では、飲酒の法定年齢は20歳です。
In Japan, the legal age for drinking is 20.
成年 refers to the age of majority (adulthood) in general, while 法定年齢 is the specific age set by law for a particular activity.
Compound of 法定 (ほうてい, 'legal, statutory') and 年齢 (ねんれい, 'age').