Firstly, I'm not sure if this is the best place to post this question because there isn't quite one answer, but in my opinion what proves to be the "best" answer would be a well supported one with lots of information on different approaches.
I've been tasked with helping to redesign a math curriculum for an enrichment program and I have very few restrictions on what the curriculum should include or how it should flow. The curriculum is meant to be an enrichment curriculum (supplementary to the existing one here in Ontario).
I wanted to find some resources on proven and effective ways to design and teach math so that students have a deeper, more rigorous and more intuitive understanding of math - an approach that prepares them to tackle math abstractly and ultimately prepares them for higher level math. The curriculum starts at grade 4 and continues on to grade 11.
There are two main competing approaches, which can be termed respectively internal and external.
The internal approach holds that mathematics has inner coherence and compelling force that need to be taught to the students to give them an idea of the true nature of mathematics in its unadulterated form. Since set theory is commonly taken to be the foundation of mathematics, in this approach students are likely to see some concepts of set-theoretic type rather early on. This was the philosophy born out of a conversation between Piaget and Dieudonne and eventually gave us the New Math (sometimes called Modern Mathematics).
The external approach holds that mathematical concepts can only be taught effectively through games of association of real-life phenomena that students are already familiar with, and encourages the use of applications to motivate mathematical concepts. Proponents of this approach agree that mathematics has internal coherence but argue that such coherence (as well as advanced foundational issues) can only be appreciated by mature mathematicians after a period of training involving more hands-on techniques as above. A major proponent of this approach is Hans Freudenthal.
A fine study of this issue is this article by Christopher J. Philips:
See also the questions under this tag.