I was six. My DD who is 9 wants nothing to do with it. I also worked as a certified ear piercer for ten years. On a professional level, the best thing I can suggest is that she's able to sit still and if you can have them both done at the same time, please do. If she freaks out once in the chair, let her down. Forcing her to go through with it only stresses her out, you out, and the ear piercer out. Under one year of age is really the least stressful time to have them done because in actuality, it really doesn't hurt that much, but after a year old, they know fear.
Personal standpoint, I always said if I had a little girl, I'd do her ears before her first birthday but my DH was adament she be old enough to ask. Disaster. She asked at 8, then cried when she got into the chair and felt worse when her friends came into school with them pierced. She beat herself up for day. Sigh. I regret it to this day, not having them done that early.