My pediatrician said once babies reach about 15 lbs, regardless of age, they can have a little water. The peds of most of my friends children agree to something more or less along these lines, unless your baby is malnourished/not eating well/etc.
My daughter, a very healthy 2 year old who nursed until 21 months, started taking a cup of water around 4 months. My son, almost 5 months now, just started a few weeks ago. Obviously, moderation is key.