The Mishnah on 18a directs us to put an erekh subject in his twentieth year in the lower erekh bracket, as well as someone in his fifth year in the lower erekh bracket,based on a gezeira shava from the case of someone in his sixtieth year.

The Zevach Todah explains that although no gezeira shava exists to carry this law over to the case of an erekh subject who is thirty days old, we learn through a binyan av to make that extrapolation. My question is as follows: Zevachim 50b leaves as a Teiku the question of whether one can make an acrobatic derasha from a gezeira shava to a binyan av. But isn't that what the Zevach Todah is doing? And although that sugya in Zevachim applies only to Kodashim, isn't Erchin subsumed within the category of Kodashim?

Thank you very kindly.

Shalom Spira, Montreal, Canada