I so disagree! First of all, nursing your child is the most natural thing in the world, and there is no way you can teach bad habits through this process. 18 months old is by no means too old to do so. Sarah's mother is right -- the best thing is to let children wean themselves. It's the most natural, and best physically and psychologically for the child and mother.
Secondly, it is very rare for a mother to get "too attached" to her child. The problem these days is that parents aren't attached enough! There is no reason at all that a woman should feel pressured to stop nursing because she is getting "too attached". That bonding is very natural, and why on earth should we want to separate ourselves from our children at such a young age?
Sarah's advice from her mother and others is correct: everyone is different, and she should go with what feels best for herself and her family.