Torah Temimah cites Toras Kohanim (Sifra, Vayikra Dibbura DeChovah, Section 5 on se’iras izzim).
Sefaria — Toras Kohanim (Sifra, Vayikra Dibbura DeChovah, Section 5)
“A she-goat of the goats” — and regarding the chatas of idolatry it says “there shall be one law for you for the one who acts unwittingly”; just as there a goat in its first year, so here a goat in its first year.
Hekesh via the shared “torah achat” language that equates all unwitting sinners to the standard of the idolatry chatas.
The ordinary individual’s female goat for a chatas must be in its first year. The Sifra does not invent the age requirement from silence; it derives it by hekesh from the explicit age of the idolatry chatas, using the common formula that there is one law for every unwitting transgressor. The result is that a goat older than one year is invalid for this offering.