57. How old was Ishmael when he was abandoned by Abraham?

He was an infant.

Ishmael was a child, young enough to be carried on his mother's shoulder, and then cast under a bush when she couldn't bear to watch him die.
Abraham ... took bread, and a bottle of water, and gave it unto Hagar, putting it on her shoulder, and the child, and sent her away. And the water was spent in the bottle, and she cast the child under one of the shrubs. ... Arise, lift up the lad, and hold him in thine hand. Genesis 21:14-18

He was a young man.

Ishmael was at fourteen years old when Isaac was born. A few years later, after Isaac grew and was weaned, Ishmael was abandoned by Abraham and Sarah (with God's consent).
Abraham was ninety years old and nine, when he was circumcised in the flesh of his foreskin. And Ishmael his son was thirteen years old, when he was circumcised in the flesh of his foreskin. Genesis 17:24-25

And Abraham was a hundred years old, when his son Isaac was born.... And the child [Isaac] grew, and was weaned. ... And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking. Genesis 21:5-9