I let my 5 year old use a calculator if he asks to, but not for arithmetic homework. I will let him check his homework with the calculator, though. I have shown him how to use the exponent function and a bunch of other stuff. Actually, the calculator itself has led to math discussions--e.g. What does this button do? He wants to use my fancy-schmancy HP "reverse Polish" graphing calculator. My 9 year old DD shows no interest in the calculator.