。
She's still a minor.
Literal
[topic-は] still minor [copula-だ].
未成年 ('minor, underage') is a Sino-Japanese compound: 未 ('not yet') + 成年 ('age of majority'). The kanji prefix 未 ('not yet') is productive and yields a whole family of words meaning 'still not the [thing]': 未来 ('future,' lit. 'not yet come'), 未婚 ('unmarried'), 未完成 ('unfinished'). The age of majority in Japan was lowered from 20 to 18 in April 2022 for most legal purposes (voting, contracts), though the drinking and smoking ages remain at 20 — so 未成年 in modern Japan typically refers to under-18s, with the 18-19 age group in a legal middle ground for some activities.