Fetching details from the dictionary...
(one's) 40s; forties
彼は四十代の会社員だ。
He is an office worker in his forties.
四十代で転職を決意した。
I decided to change jobs in my forties.
四十代になると体力の衰えを感じる人が多い。
Many people start to feel their physical strength decline in their forties.