noun
one's 40s; forties
Refers to the age decade from 40 to 49 years old. Used for people, and sometimes for eras or periods.
彼は四十代で会社を辞めた。
He quit his company in his forties.
４０代の女性に人気の趣味は何ですか。
What hobbies are popular among women in their 40s?
Compound of 四十 (forty) and 代 (generation, era, age range).