The story of Abraham binding Isaac is told in Gen 22: 1-18. Excerpting from v. 2:
[God] said, “Take your son, your only son Isaac, whom you love....and offer him ... as a burnt offering..." (ESV)
... וַיֹּ֡אמֶר קַח־נָ֠א אֶת־בִּנְךָ֙ אֶת־יְחִֽידְךָ֤ אֲשֶׁר־אָהַ֨בְתָּ֙ אֶת־יִצְחָ֔ק …. וְהַעֲלֵ֤הוּ ... לְעֹלָ֔ה (BHS)
The midrash Genesis Rabbah states that Isaac was 37 (or 26?) years old at the time.* I had always assumed he was what we might call a "child," but I don't know why. Abraham refers to him as both בְּנִ֑י ("my son") and הַנַּ֔עַר (v. 5; ESV, "the boy"; LXX, τὸ παιδάριον). I would imagine that בֵּן ("son") can be used to refer to a person of any age, but I'm not sure if נַ֫עַר denotes any particular age range.
The beginning of the story also doesn't give me a lot of insight into its timing in the life of Isaac:
וַיְהִ֗י אַחַר֙ הַדְּבָרִ֣ים הָאֵ֔לֶּה ("After these things..."). It's unclear how long "after" or exactly what "these things" refers to.
Is there anything in the text to help us figure out how old Isaac was at the time?
*The reasoning is explained in footnote 5 (p. 497) in that link, but it relies on a tradition not evident in the text to my knowledge.