I really needed to hear this, thanks! We are in Ontario, Canada, where my DD is always one of the youngest in her "grade" (this doesn't mean much because we HS, but the kids still love talking about what grade they're in anyway, lol). I went to school in the US though and I know that 6 was always considered a 1st grader!
And thanks so much everyone for the suggestions again. I only really knew Saxon/Singapore/Math Mammoth as options, so glad to know there's a lot of good curriculum out there.