I remember listening to an episode of The Science Show podcast where the guest was arguing (based on their experience) that chemistry and physics concepts should be taught to young children as it's actually easier to teach it to them. With the stated reason being that if they're taught it at a later age, as they are now, they've already built a view of the world where those concepts are somewhat unintuitive. If they learn the concepts earlier they are more easily absorbed.

I found their point fairly compelling, and I wonder if it applies to mathematics as well.

The episode was http://www.abc.net.au/radionational/programs/scienceshow/you...

One of the most fun things I ever did with my five year old son (now six) is build a cloud chamber with dry ice and a fish tank and talk about it. Strongly recommend.