My first pair of heels was for a church thing and I was like 12 and they were like maybe 2in but I said 13-15 because I was fifteen when I bought these really cute ankle booties that I adore and they're about 3-4 inches. I bought these pair of professional heels that are nude and super cute and 4in at age 16 and then at 16 (later that summer) I bought these really cut coral wedges that are around 5in. I'm 17 and I don't wear heels super often but I know the tricks. Heels that are the most comfortable are the ones where your foot isn't forced to be arched, more comfortable shoes have a platform for the front of your foot.
So I was at a hotel the other day and I guess they there was beauty pageant... I saw this adorable toddler with her pacifier and she had her little heels on!!! It was soo cute. I've been wearing heels since I was 15. Which is why I am can stay standing in 7" X) I believe it is better to learn how to walk in them while you are young, but only if the girl is fully grown horizontally.