不算年輕的，年紀不輕的，中年的
牛津詞典的解釋是“if you talk about a person being of a certain age, you mean that they are no longer young but not yet old”
柯林斯詞典的解釋是“of an unspecified age, but no longer young”（年齡不詳，但不再年輕），可以了解為“到了一定年紀的...”
經濟學人例句
But, for single people of a certain age, there have also been awkward questions.
但是對于到了一定年紀的單身人士來說，也有一些尴尬的問題。
例句
The song “Yi Jian Mei” , or “A Twig of Plum”, is the theme song of a 1980s TV drama series. To any native Chinese person of a certain age, the tune is unmistakable.
《一剪梅》這首歌是上世紀80年代電視劇的主題曲，對于任何一個上了一定年紀的中國人來說，這首歌的曲調再熟悉不過了。
