Rashi on Numbers 15:11/רש"י על במדבר ט״ו:י״א

1או לשה גו׳ [THUS SHALL IT BE DONE FOR ONE OX …] OR FOR A YOUNG SHEEP שה בכבשים או בעזים — i.e. whether it (the שה) be of the lambs or whether it be of the goats. כֶּבֶשׂ and שֶׂה are the names given to sheep or goats within their first year., אַיִל is [the name given] from the age of thirteen months and one day (Mishnah Parah 1:3).