What age/Grade level are the courses not marked with grade level?

For reference, I am homeschooling a 10 year old and have noticed some courses, specifically US History, among others, are not specified to a certain grade level.  The history introduction probed thought processes too advanced for her level and therefore wasted time.  Where can I find the targeted grade level for unleveled courses?  If unavailable, is there, at least a list of required prerequisite for each course?  If so, where can I find this information?