Albert Barnes Commentary Exodus 12:5

"Your lamb shall be without blemish, a male a year old: ye shall take it from the sheep, or from the goats:" — Exodus 12:5 (ASV)

Without blemish - This is in accordance with the general rule (margin reference): although in this case there is a special reason, since the lamb was in place of the firstborn male in each household. The restriction to the first year is unique, and refers apparently to the condition of perfect innocence in the antitype, the Lamb of God.