Their norms are by grade, not age, so you have to pick one... but I don't think they actually check that you're doing it the way they'd like or anything... and since we're homeschooling there isn't anyone else for them to ask in our case!
I first signed DS up when he was 7 as a 3rd grader -- could be once- or twice-skipped depending on whose cutoffs you use (that is, our local PS would have admitted him as a 2nd grader that year, but the private schools would have wanted him in 1st). Nobody blinked except the other parents waiting in the school library where he took it. LOL We've increased his grade at the rate of one per year without regard to what work he's doing... which probably isn't strictly fair to the other kids, but then it's the same issue as Dottie's above -- there isn't a grade that I could choose without putting it in quotes for one reason or another!

I'm not going to worry about it, because this is our last year with them anyway and we'll be doing the ACT on our own without interfering with anyone else's system. I might have felt guilty if I waited until this year to sign him up as a 3rd grader -- he would have been both at the very upper end of the possible age range AND tutored way ahead of that level.