This Bible contradiction is from the Skeptic's Annotated Bible.
He was an infant.
Genesis 21:14-18 View context
And Abraham rose up early in the morning, and 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 she departed, and wandered in the wilderness of Beer-sheba.
And the water was spent in the bottle, and she cast the child under one of the shrubs.
And she went, and sat her down over against [him] a good way off, as it were a bowshot: for she said, Let me not see the death of the child. And she sat over against [him], and lift up her voice, and wept.
And God heard the voice of the lad; and the angel of God called to Hagar out of heaven, and said unto her, What aileth thee, Hagar? fear not; for God hath heard the voice of the lad where he [is].
Arise, lift up the lad, and hold him in thine hand; for I will make him a great nation.
He was a young man.
Genesis 17:25 View context
And Ishmael his son [was] thirteen years old, when he was circumcised in the flesh of his foreskin.
Genesis 21:5 View context
And Abraham was an hundred years old, when his son Isaac was born unto him.