It matters not whether they were the first-born, or others; it is sufficient that they were persons appointed and authorized for the present service, not without God's direction. Peace-offerings of oxen; one kind, as the principal is named for all; for there were offered also goats, as appears both from Hebrews 9:19, and from hence, that burnt-offerings were usually made of the goats, Leviticus 1:10 Numbers 7:28.
Related Commentaries of Exodus 24:5
Exodus 24:5
5 And he sent young men of the children of Israel, which offered burnt offerings, and sacrificed peace offerings of oxen unto the LORD.