Personally, we nightweaned dd at around 20 months simply so that I could get more sleep--in a perfect world, I would have nursed her on demand indefinitely. But we felt that she was ready to cope with this change, and the sleep deprivation was really taking a toll on our family. (Of course, I still cried over it!)
I think it is a very personal decision and you have to figure out what is right for your child and your family.
You might want to look over on the nightwaking board...there is a LOT of discussion of this topic!