The ‘aqiqah is sunnah. Imams Ahmad and Tirmidhi related a hadith which Hakim made tashih of on the authority of Samurah that the Prophet (sallaAllahu alayhie wa sallam) said,
“The child is bonded by its ‘aqiqah, slaughter for it on the seventh day, shave its head, and give a name.”
Its time begins at birth, and if done before birth it does not suffice. The preferred time is the seventh day. The day of birth is counted from the seven, and if a baby is born at night then the next day is the first of the seven.
According to the Shafiyyah, the ‘aqiqah does not expire; and it should not be delayed until then. If it would be delayed that long, the father’s duty to perform it falls away. The child then has the option to perform it for itself.
In Rawdah, Imam Nawawi related from Bushanji that if the ‘aqiqah is not performed on the seventh, then on the fourteenth, and if not, then the twenty-first. In Sharh al-Sunnah, Baghawi related this from ‘A’ishah (radyAllahu ‘anha).
In Kifayat al-Akhyar, it is stated that it is best it not go past the duration of nifas; and then that it not done then, for it to not go past the duration of breastfeeding; and then that if not done then, then to not go past seven years; and then that if not done then, then to not go until puberty. And if it does, then the father’s duty falls away.
What suffices for ‘aqiqah is the same as that which suffices for udhiyah. The least of which is one complete sheep or goat or a seventh from a cow or camel.
It is permissible to share a cow, for example, between children or with a group slaughtering for different reasons. In Sharh al-Muhadhdhab, Imam Nawawi mentioned,
“And if one slaughters a cow or camel for seven children or shares in it with a group, that is permissible regardless as to whether all of them intend ‘aqiqah or some ‘aqiqah and some just for meat. This is like what was previously mentioned pertaining to udhiyah.”
In seems like a boy would require two shares if slaughtering a cow, in Hashiyat al-Sharwani,
“[The matn’s wording is ‘with two shahs’] and like two shahs is two sevenths from the like of a camel.” (Qalyubi)
Qalyubi also mentioned that to do more is best, to slaughter more sheep (mentioning 3 to 7), or a cow, or camel. Preferring a cow or camel is also mentioned in Rawdah,
“Some of the As-hab mentioned that the ghanam is better than the camel or cow, and the relied-upon opinion is the other way around, similar to udhiyah.”
And Allah knows best.