underage
adjective uk/ˌʌn.dəˈreɪdʒ/ us/ˌʌn.dɚˈeɪdʒ/
未满法定年龄的;未成年的
younger than the lowest age at which a particular activity is legally or usually allowed
There are laws against underage sex and underage drinking. 有禁止未成年性交和饮酒的法律规定。
He was prosecuted for having sex with a girl who was underage. 他因和未成年少女发生性关系而被起诉。
Opposite: overage