All the literature says 12 months. However, I gave DS tap water from about 8 months. I figured that as he used to chew the handle on the Tesco's trolley (yuk yuk)and stay healthy that tap water was hardly likely to cause any problems.
The food standards agency says you only need to boil tap water for babies under 6 months old although I only found that out recently and had boiled it for dd for most of her 1st year - could have saved myself some hassle...lol
FSA says: For babies under six months old, take water from the mains tap in the kitchen and boil it.