All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is not sufficient for you to give the value of ‘Aqeeqah (a sacrifice offered on the occasion of the coming of a new born) as a charity instead of slaughtering it, since the Prophet said: “A boy is still mortgaged by his ‘Aqeeqah until his guardian offers it on the 7th day, gives him a name and shaves his head.” [Ahmad, At-Tirmithi, Abu Daawood, An-Nisaa-i, and Ibn Maajah from Samura; At-Tirmithi graded it as authentic]
The expression "Mortgaged by his ‘Aqeeqah" means such an ‘Aqeeqah will free him from the harm that he received from the Devil's stab when he was delivered.
Thus, ‘Aqeeqah is to be offered on the 7th day, if it is not offered on that day, it should be offered on the 14th day or the 21st day. Otherwise, it is still a Sunnah to be offered even when one reaches the age of puberty. Here, one is required to offer his own ‘Aqeeqah.
Know that ‘Aqeeqah on behalf of children will entitle their guardian on the Day of Judgment to receive their intercession if they die before the age of puberty.
Allah knows best.