John Gill Commentary Exodus 24:5

"And he sent young men of the children of Israel, who offered burnt-offerings, and sacrificed peace-offerings of oxen unto Jehovah." — Exodus 24:5 (ASV)

And he sent young men of the children Israel
To the altar under the hill he had these young men, according to Jarchi, were the firstborn of the children of Israel; and so the Targums Onkelos and Jonathan; and the latter adds, ``for until this hour the worship was among the firstborn, yet the tabernacle of the covenant was not made, and yet the priesthood was not given to Aaron.''

But though this is a notion that has obtained among learned men, both Jews and Christians, it has been called in question by some, who have reasons against it that are not easily refuted. And very probably, as the seventy elders were selected from the senior and graver part of the people, so these were choice young men, that were separated from others for this service, without any regard to birthright:

which offered burnt offerings, and sacrificed peace offerings of oxen
unto the Lord ;
by way of thankfulness; and such were used at making covenants, when the parties ate and drank together. The Vulgate Latin version has it, "twelve calves", without any authority for it.