The phrase "a certain age" is an idiomatic expression that refers to a specific period or stage in someone's life. It can be used to describe a person who has reached a particular point in their development, whether it be physical, emotional, intellectual, or social. The term can also imply a level of maturity and experience that comes with having lived through certain experiences. In essence, "a certain age" is a way of referring to someone's stage of life without being too specific about their exact age.