In the absence of other food intolerances/allergies, anything should be fair game after six months of age (with the exception of milk and honey of course). When you do decide to try beans (highly recommended from a nutrition standpoint!), here are some things to try in the preparation to aid digestion:
I would give the beans a good long soaking, a thorough rinsing and add a 1" piece of kombu (a sea vegetable) to the cooking beans. Kombu is high in glutamic acid which helps break down the raffiose (a sugar in the beans) our bodies (and flora) have a tough time digesting sometimes. Also, don't salt the beans until they're done cooking. Puree and thin with some breast milk or water.