Among the specifications of a Qurbani animal, is the age of the animal. Just as the animal must be from the category of animals permitted for Qurbani, it must also be of the prescribed age. Should an animal not meet the age requirement, it will not be permitted for Qurbani. Such Qurbani will not be valid, and will have to be repeated.
The age requirements are as follows:
CAMELS - 5 years old or more
CATTLE - 2 years old or more
GOAT AND SHEEP* - 1 year old or more
*If the sheep is six months and older, but it is so healthy and well-built that it resembles a one-year-old, then such a sheep will be permitted for Qurbani. The way to determine this is if the 6-month old sheep is placed within a year-old flock, it does not appear smaller than the rest of the flock. This age concession has been granted only for sheep, not for goats or any other Qurbani animal.
If the sheep is younger than six months, but resembles a one-year-old sheep it will NOT be valid for Qurbani. The concession strictly applies to sheep that are 6 months or older.