What's the youngest age for a flowergirl to be for a wedding?

I really want to have my niece as my flowergirl but she will only be two and half years old. I'm afraid that she won't be old enough by then to be in the wedding. She is unpredictable, you never know what she's going to do. Whether she'll go off and just blab nonsense (all cheerful and cute) or if she'll get scared and burst into tears from all of the guests. I honestly don't even know if she will do it. If by some chance she can't do it, my cousin's three year old can. The problem with that is, that my sister's already upset that my cousin is my maid of honor and not her... but then to go and change the flowergirls, she will be so angry! My sister keeps saying yeah that she'll be able to do it, but I believe that she is only saying that to see her daughter in the wedding. I really don't want any distractions and I want the wedding to go smoothly, but I don't know what to do. I really don't want my niece to start crying during the "any objections" part but really want her to be in it.