How old was Isaac when Abraham took him to be sacrificed?

We are not told how old Isaac was at the time he was taken to be sacrificed on Mt Moriah (Genesis 22:1-19).  He was old enough to walk for three days, and to carry the wood for the burnt offering (Genesis 22:6). So it would seem he was probably at least 12, and possibly considerably older.

We also know that Isaac was 40 when he married Rebekah (Genesis 25:20), and it would appear that this took place at least a few years after the sacrifice on Mt Moriah. So an upper limit on Isaac’s age at the time of the sacrifice is about 35.

Beyond that we can only speculate. Some people have imagined that Isaac was about 33 because that is how old Jesus was when he was sacrificed. So it would be neatly symbolic if Isaac was about the same age. But there is no real evidence for the suggestion.