。
My daughter will come of age next year.
Literal
Daughter [topic-は] next-year adulthood [to-に] reaches [polite-ます].
成年に達する = 'reach adulthood.' 成年 ('legal adult age') is a formal word. In Japan, the age of majority was 20 until 2022 when it was lowered to 18. Adulthood is celebrated with 成人の日 (Coming-of-Age Day) in January.