It may seem surprising to learn that the rabbinical consensus on this, although there is certainly more than one opinion about it, tends toward the idea that Isaac was 37. This is computed on the presumption that Sarah, who is not heard from again after this event, must have died at the thought of losing her beloved son.
According to the Rabbis, the 'aḳedah not only coincided with, but was
the cause of, the death of Sarah, who was informed of Abraham's
intention while he and Isaac were on the way to Mount Moriah.
Therefore Isaac must then have been thirty-seven years old (Seder
'Olam Rabbah, ed. Ratner, p. 6; Pirḳe R. El. xxxi.; Tanna debe Eliyahu
Another ancient Jewish opinion is offered by Josephus, who stated:
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. (Ant. 1.13.2).
Among Christian commentators opinions tend to agree that Isaac was not a little boy.
Adam Clarke said: “[I]t is more probable that he was now about
thirty-three” (1:140, emp. in orig.). Jamieson, Fausset, and Brown
asserted that Isaac was “then upwards of twenty years of age” (n.d.,
p. 29). J. Curtis Manor described him as “a youth of sufficient
strength and agility to carry a load of firewood up a mountainside”
(1994, p. 103). Keil and Delitzsch affirmed that “this son had grown
into a young man” (1976, 1:248). Morris added: “[T]he meaning in
Isaac’s case should also be ‘young man’ ” (1976, p. 373).
Objections to this relatively mature age based on the word נַעַר (lad/youth) may be overcome by the fact that Isaac was still single, and Hebrew tradition in ancient times did not consider that full manhood was reached until marriage.
Despite the typical depiction in Christian art, in which Isaac is usually depicted as a boy, Jewish and Christian commentators alike recognize the likelihood that he was actually what we, in the modern world, would call a full-grown but not yet married man.