legal age
Refers to the age at which a person is legally permitted to do something, such as drink, vote, or marry.
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').