How old was Abraham when Ishmael was born?

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86 years old

And Abram [Abraham] was fourscore and six years old, when Hagar bare Ishmael to Abram. Genesis 16:16

More than 135 years old

Abraham's father, Terah, was seventy years old when Abraham was born.

And Terah lived seventy years, and begat Abram. Genesis 11:26

Abraham didn't leave Haran until Terah died.

Our father Abraham ... dwelt in Haran: and from thence, when his father was dead, he removed himself into this land. Acts 7:2-4

And Terah died when he was 205. .

And the days of Terah were two hundred and five years; and Terah died in Haran. Genesis 11:32

Sometime after leaving Haran (when Abraham was at least 135 years old), Ishmael was born.

And he [Abraham] went in unto Hagar, and she conceived ... And Hagar bare Abram a son: and Abram called his son's name, which Hagar bare, Ishmael. Genesis 16:4-15