I think there's a lot to consider to be honest. I never wear heals due to my height. In high school I would have been teased for being too tall ( more than I already was). My 6 year old constantly asks for them and is told no (mostly due to her behavior) but we also need to consider her weak ankles. She is clumsy at the best of times and a simple twist could be enough to give her a severe sprain.

If her behavior and body (strength, height etc) where 97%, I still wouldn't let her until she was going to her formal...and even then they would be thick based to give maximum stability