I was pretty annoyed when someone gave it to my three year old. I would prefer they didn't have it until they were about six. But then, we don't normally drink it at home, so I suppose if they have it it will just be something they just have at parties so it's not such a big deal.
My kids were around 6 when they had it for the first time. They drink it now on occasion, but certainly not on a regular basis. My husband drank Coke from a very young age and I think that is just awful!
You know what? If I had my time over again as a parent I don't think I would introduce soft drinks at all. Compared to other mothers I was extremely conservative with soft drinks and my child was school age before she could have the odd glass on special occasions. I think that my child being cavity free for her entire childhood proves that it was a good decision on my behalf. Soft drinks are empty calories, full of sugar which lead to all sorts of potential health issues.