The current recommendation for peanut butter is one year, unless there is a history of allergies in the family, in which case you are advised to wait until 3 years. A lot of the Annabel Karmel recipes use peanut butter and it's an excellent source of protein and vitamins for vegetarian toddlers.
Incidently, the UK guidance recommendation that pregnant women and children under 3 shouldn't have peanuts is soon going to be changed. In Israel, nearly all babies are weaned onto peanut paste and they have one of the lowest incidence of peanut allergies in the world. The latest evidence suggests that early exposure actually reduces the chance of allergies.
But as the others have said, the potential risks can be pretty serious. If you do decide to introduce it, use a small amount at first and keep a close eye on your child for a few hours afterwards.