We visited when my daughter was just shy of 4, and my SIL did her hair in our room somewhat fancy and got some hairspray glitter and lent her a little rhinestone mickey clip and she felt like a million bucks.
I just was unsure about the value and her tolerance for it, and at that age she didn't really care some girls had a sash. We did CRT that day and popped her dress on right before the meal and everyone made such a big deal out of her being a princess we were really glad we didn't pay $$$ and take time for it - the make her feel like a princess in the castle thing was covered, and that was the experience we wanted anyway. At that age, her idea of dress up was maximum glitter and minimum time.
We visited when my daughter had just turned 6, and I talked to her about it before, and she really wanted to do it. We had a really special time, she understood that we'd have to wait (we waited 30 minutes after our appointment time before we got seated), how to sit still, that they might pull on her hair a bit, etc. She was old enough to appreciate all the stories and fun extra that the fairy godmothers put into it. I had FP booked immediately after for her and I to visit the princesses, so we had a bit of girl time in the park and the princess visits were fantastic and she felt really special again. We put those colored hair things and mickey bobbie pins in her ponytails the rest of the trip, and I really felt like it was worth it. This time around she definitely noticed the sash and the bag (she wouldn't let that bag go out of sight!). This time she knew that people get manicures and she was getting one too, that makeup goes on your eyes, that the end result was worth tight ponytails, etc.
For us this 5-6 window was the right one. I'm glad we didn't do it on the first trip - I think she would have had a meltdown in the waiting room alone (not to mention probably pulling her hair, sitting still issues, etc.). This trip was all around awesome, she was mature enough to handle waiting and momentary discomfort but young enough to still really get into it. Next trip (in a year) I'm planning on pirates makeovers since I saw a little boy get his thing done in BBB and it was so underwhelming and my son was just-barely-not-jealous of sister and they both loooooove pirates.
Your daughter may be different though - just think through the $$$ and experience and time and her capabilities and have fun. I don't think there is a right age - just a right age for your kiddo.