Bechorot 4: When Did the Bechorot Become Sanctified?

Were animals sanctified from the time that the Jews were in the desert?  Or was it later in time that the animals became sanctified?  Rabbi Yochanan quotes Shemot (13:2) where one is required to sanctify the firstborn of humans and animals and thus concludes that the laws of bechor happen in the desert.  Reish Lakish argues that the end of that passage in Shemot teaches that the laws of bechor only apply when the Children of Israel are within the Land of Canaan (13:11-12).

The Rambam interprets Shemot (13:11) as saying that the firstborn are sanctified if they were alive at the time of the Exodus. This celebrated that they were saved while the Egyption firstborn were killed in the final plague.  Regardless, the obligation to apply this sanctification to future generations did not  take effect until after the journey into the desert had ended. 

Rabbi Yochanan debates: a barita states that before the Tabernacle was built, the sacrificial service in the desert took place on private altars by the firstborn.  Reish Lakish retorts that this only refers to the firstborn who were already alive at the time of the Exodus.

Steinsaltz teaches us that the sacrificial service in the desert performed by the firstborn was based on a passage in Shemot (24:5) where we learn that when Torah was given on Sinai, there were a group of "young men" who brought sacrifices.