Some might surprise you. I brought my Altair over to a friend's house and his clever 5th grade son was enthralled by it. I explained binary numbers and how to convert from binary to octal. Then I taught him some basic 8080 assembly instructions and he was entering his own programs on the front panel shortly later. I wrote out a basic program for adding two 8 bit integers and without my explanation, he figured out converting decimal <-> octal and chose some numbers you might expect from a 5th grader, like adding 6 and 7, and choosing two numbers that summed to 67 in decimal.