Yishmael’s Bris

Yishmael was born to Avraham when he was 86 years old. Avraham performed the Bris Milah on Yishmael when Avraham was 99 years old [Bereishis (Genesis) 16:16, 17:24].

Question: How old was Yishmael when he received Bris Milah? 

Answer: 99 years – 86 years = 13 years old