Kids still need to learn how to add, subtract, multiply, and so on - BEFORE they pick up the calculators! If we start giving them calculators from age 5 on, they're not doing anything but going through the motions - learning which buttons to push. There's no active construction of mathematical knowledge going on there. We all should know that: I purposely use calculators because it makes my life easier. And I pay for it too - after years of having technology do it for me? I'm a lot slower at simple mathematical operations than I used to be.
Imagine if this was the only way we ever had done math: how would we ever continue to advance mathematical knowledge?