תורה תמימה
TORAH TEMIMAH
שמות פרק ל, יד כֹל הָעֹבֵּר עַל-הַפְׁקֻדִּים מִּבֶן עֶשְׁרִּ ים שָנָה וָמָעְׁלָה יִּתֵּן תְׁרוּמַת ה'
Rabbi Akiva Eiger (או"ח סימן ק"ו) states that although women are obligated in formal prayers, they are exempt from the Musaf prayers. Musaf prayers only commemorate Korbanos Tzibur which are brought with the half-shekel, which are not taken from women.
If this is correct, then young males under 20 should also be exempt from Musaf and could not be the Shliach Tzibur to lead the prayers. However, these days the Shliach Tzibur does not fulfill the obligations for those that listen; rather we have a person repeat the prayers only due to a previous custom. Thus, it is possible for a person younger than 20 to lead the Musaf prayers.