My boys both started proper lessons at about 3 - 3 and a half. I agree with toby, although they both really enjoyed the lessons and had great water confidence, they didn't actually swim before they were about 5-6. They had the technique, just not enough strength to pull themselves through the water for long enough and the kids that started later soon caught up. By the time they could swim, they were comfortable enough to swim underwater and dive to the bottom of the pool - turn underwater somersalts etc.
It will certainly cost you a lot more money in lessons starting earlier for essentially the same result, but it really depends on what you expect from the lessons.
If she enjoys them and you can afford it, I would say go for it - it is great exercise and being confident in the water is great for them and as an abbsolute minimum, she will be able to keep herself afloat from a young age if not necessarily get across the pool!