ANSWER: The Biblical story of Abraham and his attempt to offer his son Isaac as a sacrifice to God is a fertile subject in Jewish literature and tradition. While some statements regarding this epic event (known as the akedah in some writings) have some truth to them, others simply cannot be supported in the light of a proper reading of the scriptures. One example of this is a tradition that says it was Satan (and not God deciding to do so on his own) who suggested that Abraham be tested by requiring him to sacrifice his only son.
In regard to how old Isaac was when he was to be sacrificed, one tradition says the event occurred very soon after he 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 Isaac was weaned at two years old; Gedaliah says three; Jerome says some Hebrews believe he was five; Philo makes him seven; and Bishop Usher places the weaning at ten or twelve years of age. An average of all these opinions would place the weaning at about six years old.
The Bible stories in children's books must terrify little boys and girls as they generally depict Abraham's son as a small boy when he was about to be sacrificed.
A tradition among some Rabbis is that the news of Abraham's journey to Mount Moriah and its real purpose caused the death of Sarah, his wife. From this, they deduce that Isaac had to have been at least thirty-seven years old when he was to be offered up to God.
The first century Jewish historian Josephus also took a stab at how old Isaac was when his father was commanded directly by God to sacrifice him. In his huge volume tracing Jewish history from creation forward, he states the following.
"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 (History) of the Jews, Book 1, Chapter 13)
Abram (the original name of Abraham before God changed it in Genesis 17:1 - 5) lived in the land of Palestine (the promised land) "many days" (verse 34). In Biblical terms a day often refers to a year, so how many years Abram lived there is anyone's guess, but "many" days (years) would likely indicate at least ten. However, we do know it was "after these things" that God told Abram to sacrifice his only son (Genesis 22:1), which would mean he was possibly sixteen years old or older when they left for Mount Moriah.
It is almost certain that Isaac had to be teenager or older, and not a small boy, when he was to be offered. This is because Abraham not only placed on his back the wood needed for the sacrifice, he also made him carry it both TO and UP Mount Moriah (Genesis 22:2, 6). The strength and stamina required to carry this sacrificial load would be something a small boy would be hard pressed to do!