My daughter is six, knows her orange belt form so well the other kids (even the high oranges) often watch and follow her during form practice and has the three tips usually required to take the belt test to move to the next level. She has work to do and more to learn (as well as strength to gain) in sparring. However she was invited to take the "high orange" belt test even after only being in her orange belt for 4 weeks.
My husband and I feel she is being sped through very quickly, but would rather skip the "half belts" if possible, even if it means she is in each belt for longer and know of a few other kids that are doing the same thing. However, we have been told skipping half belts is not allowed before age seven. This makes me wonder what the over-arching policies are vs. the policies at just our own studio.
Based on what I see in this answer, it is mostly guidelines, but I wondered if things applied differently for the very young.