Exodus 24:5

He sent young men So also the Septuagint, τους νεανισκους : but the Chaldee renders it, the firstborn. The firstborn were priests in every family; but, among the Israelites, they were afterward redeemed, and the tribe of Levi given to God in their stead, for his service. But the Hebrew word נערי, nagnarei, here used, signifies active as well as young, and as these persons were employed in slaying and offering up the oxen, strength and activity were necessary. As Moses himself performed the principal office of the priest, (Exodus 24:6,) it cannot be concluded from this passage that these young men were priests. But it is worthy of observation, that even after the appointment of Aaron's family to the priesthood, no man was permitted to serve at the altar after the age of fifty, see Numbers 8:25.