Lambs for sacrifices are one year old and rams two years old. All these years are counted from day to day (i.e., from the animal’s birth, as opposed to calendar years). A sheep that is thirteen months old cannot be used, neither as a ram nor as a lamb. Rabbi Tarfon referred to such an animal as “palgas” (in between stages), Ben Azzai called it “noked” (belonging in the pasture) and Rabbi Yishmael called it “prachadigma” (something awaiting new validation). If a person offered such an animal, he must bring the libation of a ram with it but it doesn’t fulfill his obligation to bring an offering. Once it’s thirteen months and one day old, it’s a ram.
Communal sin offerings and burnt offerings, individual sin offerings, and the guilt offerings of a nazir (nazirite) and a metzora (“leper,” but not really) are valid from the animal’s thirtieth day and on, inclusive of the thirtieth day. If they were offered on the animal’s eighth day they are valid. Vow offerings, freewill offerings, firstborn animals, animal tithes and the Passover offering are valid from the eighth day and on, inclusive of the eighth day.