代
文法の解説は英語です。
用法
The suffix 代 is used with ten or its multiples to describe someone's approximate age, or with ten or its multiples and 年 to refer to a certain decade.
例文
彼は20代で会社を作り、30代で、大金持ちになった。
He founded a company when he was in his twenties and became rich when he was in his thirties.