My son is 6 and JUST hit 40". He's in the negative 5th percentile, though. A short little guy. I'm 5 feet tall on a good day, so it doesn't surprise me. All of his friends are a good bit taller than him and always have been. I think unless she's abnormally small like my DS, she'll be fine. I think the chances of her being under 40" are pretty slim. In fact, from what I see of my DS's friends, I bet she's at least 45", if not taller.