What are the reasons for the differences among the four schools of thought regarding the determination of the age of the "jadh'ah" (yearling) and "thaniyyah" (two-year-old) for sacrifice, and did any of the jurists explicitly state the core of this disagreement, and is it possible to reconsider the age determination based on biology, and is the age requirement for aqiqah similar to that for udhiyah, and what is the validity of the fatwa of a Shafi'i scholar that an aqiqah with a goat under two years of age is invalid?

The jurists have agreed that a sacrifice (Uḍḥiyah) is not valid unless it is a thaniy of camels, cattle, or goats, or a jadhaʿ of sheep. However, they differed on specifying the age of each.

According to the Hanafis and Hanbalis, a jadhaʿ of sheep is one that has completed six months. According to the Malikis and Shafi'is, it is one that has completed one year.

As for the thaniy: for goats, it is one that has completed one year according to the Hanafis, Hanbalis, and Malikis, and two years according to the Shafi'is. For cattle, it is one that has completed two years according to the Hanafis and Hanbalis, three years according to the Malikis, and two years according to the Shafi'is. For camels, it is one that has completed five years according to the Hanafis, Hanbalis, and Malikis.

The reason for their difference is that the determination of age (thaniy and jadhaʿ) used to depend on natural signs (such as the falling of teeth or the sleeping of wool), which varied according to different environments and conditions. Thus, each school of thought strived to establish an age-based criterion.

The conditions for the ʿAqiqah (sacrifice for a newborn) are the same as the conditions for the Uḍḥiyah in terms of age and the animal's freedom from defects.

If a goat is sacrificed for ʿAqiqah and it is less than two years old, it is valid according to those who hold that a thaniy of goats is one that has completed one year.