Learning that stuff is much easier when you're very young (just like learning new languages is). It's boring, but without that you're handicapped latter on.

Do you actually have evidence for that, or is it one of those "everybody knows" sorts of things?

And, frankly, is the the issue even relevant to whether it should be taught to kids in schools?

Seriously, when I was a kid (elementary school, jr high) I did a lot of peer-tutoring in math, because I witnessed for myself how damaging most math classes were to my peers' mathematical intuitions (which in young kids starts out pretty good). I seriously mean to say that the majority of those classes were egregiously counterproductive, ingraining in kids terrible habits of mind and attitudes about the behaviour of abstracts, which greatly interfered with their ability to pursue real mathematics.

I really don't think that, even if arithmetic is more easily learned when very young, we can trust the classroom to impart that skill without savaging all but the brightest students' facility for true mathematical abstraction.