Tosefta על חולין 1:6

[What is] valid for the red heifer is invalid for the heifer whose neck is broken (Deut. 21:1-4), and what is valid for the heifer whose neck is broken is invalid for the red heifer (Hul. 1:6). And work [in the field] renders each of them unfit. B'nei yonah (young pigeons), once they are old enough to sip (see Hul. 22b:6), until their feathers become golden [are fit for sacrifice], [while] turtledoves, once they are able to fly, even if they are old [are fit for sacrifice], and consequently it is said, what is valid in turtledoves is invalid in b'nei yonah, and what is valid in b'nei yonah is invalid in turtledoves.

Kohanim that reach puberty, and even when they grow old, are fit [to perform the Temple service], but a blemish renders them unfit. Levites from 30 to 50 years old are fit [to perform their duties], but a blemish does not render them unfit, and consequently it is said (Hul. 1:6), "[What is] fit for a Kohen is unfit for a Levite, [and what is] fit for a Levite is unfit for a Kohen." In what case does this apply? In the Tent of Meeting in the desert, but in the eternal House (i.e., the Temple), the Levites are only rendered unfit as to their voice (i.e., participating in the Levitical chorus). [What is] fit in the Kohen Gadol is unfit for an ordinary Kohen, [and what is] fit for an ordinary Kohen is unfit for a Kohen Gadol. [What is] valid in the golden vestments is invalid in the white vestments, [and what is] valid in the white vestments is invalid in the golden vestments (see Yoma 7:4).