This is the 2844th most frequent Chinese word.
成年
"成年" means "to come of age" or "adulthood."
In this sentence, '成年' is used as a verb meaning 'to come of age' or 'to reach adulthood'.
他在十八岁时成年了。
He came of age at eighteen.
Here, '成年' is used as a noun referring to the state or stage of adulthood.
成年后，他开始了独立的生活。
After reaching adulthood, he started living independently.
In this example, '成人' is used as an adjective meaning 'adult', derived from '成年'.