I've made some for my DDs, the little one is 18 months and she kept it on and my oldest is 3 1/2 and she just loves them. I've also made some for my nieces following polychic's idea about the length and tying them back on a ponytail and she LOVES it. My niece is 11 yrs old.
I would say that about 20 months up to high school age girls would be okay. Every little girl is different. If you show different age groups, you won't limit your sales and get a broader customer base