How old was Isaac when he was to be sacrificed?
The Bible does not give a specific age, but there are events surrounding Abraham's sacrifice of Isaac which gives us clues. Most scholars believe that Isaac was in his late teens or early twenties at the time of the offering.
At the time that Isaac was conceived, we are told that Abraham was 99 years old, his wife Sarah was 90, and Ishmael was 13 (Genesis 17:1, 17, 25). Ishmael was sent out of Abraham's house at the time Isaac was weaned (Genesis 21). The age of weaning varies greatly between cultures but typically it would be between the ages of 2 and 5. The next given event is a covenant between Abraham and Abimelech, but no ages are given to tell us when this occurred. It was some time after this covenant that Isaac was offered. After the sacrifice of Isaac came news of Abraham's relatives in Haran and then we are told that Sarah died at the age of 127 (Genesis 23:1).
Hence, the sacrifice of Isaac must have occurred between the age of 5 and 36, which is quite a broad span. In addition, Isaac is referred to as a "lad" or na'ar in Hebrew in Genesis 22:5 and 12. This noun is used to refer to a male from infancy to a young adult not yet married. For example, Joseph is called a lad at the age of seventeen (Genesis 37:2) and at the age of thirty (Genesis 41:12, 46).
We do have some further clues. Isaac is old enough to travel three days with Abraham but without his mother. His conversation with Abraham is that of an older child or a young adult. Finally, Isaac is the one who carried the wood for the sacrifice up the mountain (Genesis 22:6). The latter is strong evidence that Isaac was no longer a small child and serves as the main reason that our best guess is that he was at least in his late teens.