A reading from Genesis 17:25
And Ishmael his son {was} thirteen years old, when he was circumcised in the flesh of his foreskin.
— Genesis 17:25 (en_kjv)
https://peck.to/tx/ea955ebee7daf9c2aa292f99fc22db4e778d9948a39c7ab4f0f71db5da8f0574
Thirteen years old.