The term 女子 in Japanese refers to a girl or young woman, but the specific age range it covers can vary depending on the context. Generally, 女子 is used to refer to females from early childhood up to their late twenties.
In schools, 女子学生 refers to female students of any age, from elementary through university. In sports and other activities, 女子 might be used to distinguish female participants, again without strict age limits.
However, in more formal or official contexts, 女子 might be more closely associated with younger females, particularly those in the schooling age range up to young adulthood.