ANSWER: The Bible stories in children's books must terrify little boys and girls, as they generally depict Isaac as a small boy, usually ten years old or younger, when he was about to be sacrificed. We will discover, however, that this was almost certainly not the case.
Jewish tradition has much to say regarding the sacrifice of Isaac, along with how old he was when it happened. Some of their statements have partial truth in them while others are not supported at all by the Old Testament. For example, the tradition of the Rabbis claims that Satan suggested to God that Abraham be tested through his son Isaac. This, of course, is nowhere supported in Scripture.
One religious tradition says the sacrifice took place soon after Isaac was weaned (Genesis 21:8). The problem is no one is sure at what age this happened. The Jewish writers Jarchi and Ben Melech say that he was weaned at two years old. Gedaliah says Isaac was weaned at three.
The Catholic theologian Jerome says some Hebrews believed he was five, while Philo writes he was seven. Bishop Usher, the person who gave us his famous chronology of Biblical events, places the weaning at ten to twelve years of age. An average of all these opinions is that Isaac was around the age of six years old when his father was tested.
An ancient tradition of Jewish Rabbis states that the assumed death of Isaac caused his mother Sarah to perish (1906 Jewish Encyclopedia). They believe (without Biblical support) she died when she discovered, after Abraham and son left for Mount Moriah (Genesis 22:2), what was her husband's true goal that justified his travel. This logic, coupled with the fact that Sarah gave birth to Isaac when she was ninety years old (Genesis 17:17), and perished when she was 127 (Genesis 23:1), leads to their conclusion that this event happened at the age of thirty-seven.
The first century Jewish historian Josephus also took a stab at how old Isaac was when his father was commanded to sacrifice him. In his huge volume tracing Jewish history from creation to the first century A.D., forward, he states, "Now Isaac was twenty-five years old. And as he was building the altar, he asked his father what he was about to offer, since there was no animal there for an oblation . . ." (Antiquities of the Jews, Book 1, Chapter 13).
In Genesis 22:1 it states that "after these things" God tested Abraham by commanding the sacrifice of Isaac. We also know he lived in the land of Palestine "many days" (Genesis 21:34) after the birth of his son. In Biblical terms a day often refers to a year, so how many years Abraham lived there is anyone's guess, but "many" days (years) would likely indicate at least ten and likely more.
It is almost certain, however, that Isaac was a teenager, maybe even twenty years old or more, when he was to be sacrificed. We have reason to draw this conclusion since the Bible says Abraham not only placed a large amount of wood on his back for the sacrificial offering, he also made him carry it both TO and UP Mount Moriah (Genesis 22:2, 6). The strength and stamina required to carry this strenuous load would be something a small boy would not be able to handle!