My son is 11 and is finishing chapter 12 of 15 in Number Theory. I also bought the other two sets. He will begin Counting next and then Geometry.

Number Theory started pretty easy for him but as he got into it he was definitely challenged. He covered a 2+ chapters a week at first but it has slowed down now to one chapter per week.

From a brief glance at the other sets I think they will challenge him too.