Gemara:

Yevamot,  Zevachim 

Halacha:

Beit HaKenesset,  Shabbat,  Tefillah,  Yom Tov 

Holidays:

Rosh Chodesh 

References: Zevachim: 75A ,  Yevamot: 14A  

Description

If you take two assumptions (i.e.- women/males from 13-20) are not obligated in giving the Half-Shekel to buy the communal Musaf sacrafices + the Musaf-Prayer is ONLY in place of it, then perhaps a kid between 13-20 CAN'T be a Chazzan for Musaf for males over that age as he is not OBLIGATED, and one that is a volunter can't fulfill the obligation for one that's obligated.