In the West, there would probably be a genuine reluctance to admit a woman under the age of 18 into a convent or monastery (or novitiate)... probably on account that this life requires both committment and discernment of calling, whatever the religious tradition. In the East, different cultures may allow women to join the sangha at an earlier age, (esp. in Tibet) or to be raised as Brahmacarini (single, celibate, student state of life) and pursue the Brahmannical state in an ashram for women.