age forty; one's forties
Refers to the age of forty or the decade of one's forties. Often used in literary or slightly formal contexts, but also in everyday speech.
He has just turned forty.
四十路になっても、まだまだ挑戦したいことがある。
Even in my forties, there are still many things I want to try.
The suffix 路 (じ) is an old counter for years of age, seen in words like 二十路 (ふたそじ, age twenty) and 三十路 (みそじ, age thirty). The reading よそじ is a combination of 四十 (forty) and this suffix.