It doth not appear how old Isaac was, because the time for the weaning of children is very various, according to the differing tempers and necessities of children, or inclination of parents; and in those times, when men's lives were longer than now they are, proportionably the time was longer ere children were weaned.
Related Commentaries of Genesis 21:8
Genesis 21:8
8 And the child grew, and was weaned: and Abraham made a great feast the same day that Isaac was weaned.