Bible scholars have long wondered how old Isaac was when his father Abraham took him up to the mountain top to offer him as a sacrifice. Through careful study of the story as related in the Old Testament, based on the following facts: 1) Issac was old enough to understand the ritual of sacrifice, 2) Issac was old enough to carry wood for the fire to the top of the mountain, 3) Issac was old enough to notice that they were not bringing an animal for the sacrifice. Therefore Issac's age, at this time, was greater than 8 years old. Scholars also conclude that he was younger than 12 years old as supported by the following fact: If Issac had been older than twelve, he would have been a teenager and it would not have been a sacrifice.
(From the "Rest" of RHF)