This isn't for 12 year olds, it's for 8-11 year olds (grades 3-5).

My impression of this is that it's roughly on par with what is normally taught at those grades. Keep in mind that the kids aren't expected to discover anything here for themselves; they're just going to sit passively while the teacher leads them around by the nose.

Curricula change from year to year; some material get introduced earlier than it used to, while other material gets introduced later. Fractions are going out of style, for example; and continued fractions -- a standard part of A level mathematics in England in the early 1900s -- are now unheard of in high schools.

Just because something is called "mathematics", it isn't advanced. It IS possible to understand mathematics, even for normal humans.