I reached that certain age and fell in love.

我到了既定年龄却陷入爱河。

youdao

But almost one in five (19%) said that being middle age is a state of mind, rather than something that begins at a certain age.

BBC: Middle age begins at 55 years, survey suggests

Second step: They can sign a contract--symbolic, not binding--stipulating that, by a certain age (provided the business is not in financial distress) that they will move on and let the next generation take its lumps.

FORBES: Head Coach

Newt Gingrich reminds me of one of those small, ugly dogs that women of a certain age make a tremendous fuss over.

BBC: Newt Gingrich and his campaign legacy