There's no single age limit for joining a sorority. Most sororities are affiliated with colleges and universities, and their membership is generally limited to undergraduate students enrolled at that institution. Therefore, the
effective upper age limit is the age at which a student is no longer considered an undergraduate. This varies depending on the school and the individual's academic path.
Some sororities have alumnae chapters that women can join after graduation, but these are separate from the undergraduate chapters. There might be age restrictions for alumnae chapters, but they're usually significantly higher than the undergraduate age limit.