Genesis 21:8

“And the child grew, and was weaned, etc.] He throve under the nursing of its mother, and through the blessing of God upon him; and being healthy and robust, and capable of digesting stronger food, and living upon it, he was weaned from the breast: at what age Isaac was when weaned is not certain, there being no fixed time for such an affair, but it was at the discretion of parents, and as they liked it, and the case of their children required; and in those times, when men lived to a greater age than now, they might not be weaned so early, as we find their marrying and begetting children were when they were more advanced in years. The Jewish writers are not agreed about this matter. Jarchi…”

“the child grew, and was weaned--children are suckled longer in the East than in the Occident--boys usually for two or three years. Abraham made a great feast, &c.--In Eastern countries this is always a season of domestic festivity, and the newly weaned child is formally brought, in presence of the assembled relatives and friends, to partake of some simple viands. Isaac, attired in the symbolic robe, the badge of birthright, was then admitted heir of the tribe [ROSENMULLER].”

“21:8-21 God used the incident of Ishmael’s mocking Isaac to separate Ishmael and Hagar from the family and the child of promise. They would constantly threaten the promised descendant if they remained with the family. 21:8-9 The feast for Isaac’s weaning probably occurred when he was three and Ishmael was about seventeen years old (16:16). Sarah saw Ishmael making fun of her son, Isaac: The verb metsakheq (“making fun of”) is related to the word for “laughter”; this theme (21:6) is given a sour twist by Ishmael’s mockery.”