All women who are members of the Church and are 18 years of age or older and those who are married are members of the Relief Society. Therefore, young women who reach their 18th birthday will ordinarily move into the Relief Society. However, because of special circumstances, such as individual maturity, desire to continue with peer group associates, graduation from high school, and attendance at college, a young woman may move into Relief Society earlier or remain in Young Women longer. Such exceptions should occur after the young woman consults with her parents and the bishop and receives their approval.
Young Woman Handbook, p. 6