In Artscroll’s edition of Sefer HaChinuch (vol. 2, p. 303), they quote a discussion regarding whether women are exempt from davening Mussaf:
If women are obligated as men are in all the communal offerings, there is a reason to obligate women in the Mussaf prayer, since it is representative of the Mussaf offering in which women are obligated. Thus, the question of whether women must recite the Mussaf prayer today is contingent upon the question of whether they are included in the obligation of all the communal offerings, and that they are therefore exempt from reciting the Mussaf prayer (see Teshuvos R’ Akiva Eiger §9, and R’ Akiva Eiger on Shulchan Aruch, Orach Chaim 106:2).
(Artscroll spends the rest of the piece addressing how this opinion interprets Ben Buchri’s opinion that Kohanim and Leviim are exempt from the Shekalim in light of the fact that Sukkah 53a indicates that they davened an obligatory Mussaf.)
I see also that Mishnah Berurah 106:4 cites the Tzlach (Artscroll ad. loc. references Tzlach to Berachos 26a without actually quoting him) as saying similarly that women are exempt, without quoting a reason; I don’t have access to this Sefer to see his reasoning inside.
According to these opinions, that women are exempt from Mussaf due to their being exempt from the Shekalim, would the same hold true for the other groups of people exempt from Shekalim, i.e. children under 201 and slaves? (I ask more about the children, as in general slaves are liable and exempt in the same Mitzvos as a woman.)
1I base this statement on the Bartenura to Shekalim 1:3:
וקטנים. אפילו הביא שתי שערות והוא פחות מבן עשרים:
”And minors” - even if he has produced two hairs and is younger than 20.
And Yachin ad. loc.:
וקטנים כל שהוא פחות מבן עשרים קרוי קטן, ואפילו הביא ב' שערות.
”And minors” - anyone younger than 20 is called a minor, even if he produced two hairs.
However, I see that the Rambam ad. loc. understands this exemption differently:
וקטן שלא הביא שתי שערות אין ממשכנין אותו אלא שמניחין אותו ואם נתן מקבלין מידו אמרו אין ממשכנין עד שיביא שתי שערות
And a minor who has not produced two hairs, we do not take collateral from him, but rather we leave him. If he gave, we will accept from his hand. They said: we do not take collateral until he produces two hairs.
Obviously my question can only apply if children ages 13-20 are exempt from Shekalim, as children under 13 are exempt from Mitzvos anyway.