Let's take a look at what two medical authorities say about when to wean your child. The first set of recommendations from the American Academy of Pediatrics. And what they say is, "It is recommended that breast-feeding continue for at least 12 months, and therefore, for as long as is mutually desired," mutually between the child and the mother.
Let's take a look at what the World Health Organization has to say. They say simply breast-feed for up to two years or longer. So what the authorities are saying is let the mother and the child decide.
CNN.com - Elizabeth Cohen: Breast-feeding older kids - July 10, 2002