Genesis 21:7-9

Amplified Bible

⁷And she said, “Who would have said to Abraham that Sarah would nurse children? For I have given birth to a son by him in his old age.”^(A)

⁸The child [Isaac] grew and was ^[a]weaned, and Abraham held a great feast on the day that Isaac was weaned.

⁹Now [as time went on] Sarah saw [Ishmael] the son of Hagar the Egyptian, whom she had borne to Abraham, ^[b]mocking [Isaac].^(B)

Genesis 21:8 This was probably when the child was about three years of age. Samuel served in the sanctuary from the time that he was weaned (1 Sam 1:22-28), and a Hebrew mother is quoted in 2 Maccabees 7:27 as saying to her son that she nursed him for three years.
Genesis 21:9 Ishmael was old enough to know better than to ridicule or tease his much younger brother, and his behavior was inappropriate and thoughtless.