"On the eighth day shall you give it to Me": This tells me (that it is acceptable) only on the eighth day itself. Whence do I derive (that it is acceptable) from the eighth day onward? It follows, viz.: It is written here "eighth," and elsewhere (Leviticus 22:27) "eighth." Just as there, from the eighth day on, so, here, from the eighth day on. Just as here (it is acceptable) on the eighth day itself, so, there.
The Torah Specifies That a First-born Animal Becomes Eligible
Mekhilta Tractate Kaspa 1:32