How old were Isaac and Ishmael when Ishmael mocked Isaac?
Isaac was five years old and weaned when Ishmael mocked him, while Ishmael was about eighteen years old. Scripture records Sarah seeing Hagar's Egyptian son mocking, and the account describes Ishmael teasing the young child. At that time Ishmael was a teenager and Isaac was a little child.
Les explains that the account moves from Ishmael's conception by Hagar and his birth, through Hagar taunting Sarai and being sent into the desert, with the Lord sending her back. Les points out that 18 years later she would go back into that same desert, because God keeps the big picture and can leap thousands of years as though a day. Les sees this scenario as programmed by God so Paul could use it as a lesson. At the time of the mocking, Isaac was five years old and weaned, and Ishmael was about eighteen. Les notes the same attitude of rebellion and taunt, and that in Genesis 21:9 Sarah saw Hagar's Egyptian son, Ishmael, mocking, teasing the little five-year-old, as teenagers can do with little tail-enders.
And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking.