What does it mean that Ishmael was mocking Isaac?

Ishmael’s mocking meant he was taunting and teasing Isaac, who was about five years old and newly weaned, while Ishmael was about eighteen. It reflected the same rebellious, taunting attitude already seen earlier, not merely innocent play. Sarah saw the son of Hagar mocking Isaac.

Les teaches that Ishmael was about eighteen and Isaac was five and weaned. The mocking was a continuation of the same attitude of rebellion and taunt seen earlier, and it meant Ishmael was simply teasing the little five-year-old to death, like teenagers can do with the “little tail enders.” Genesis 21:9 records that Sarah saw the son of Hagar the Egyptian, Ishmael, whom he had born unto Abraham, mocking. Les also sees the whole scenario as programmed by God so Paul could use it as a tremendous lesson.

Genesis 21:9

And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking.

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.