DS is 3.5 and I generally don't hold his hand walking down the street or across quiet roads ie my entire town but if it is tourist season or if we are in a city then I do across the road. He must have been around 2.5 when I stopped holding hands everywhere, but he is not the sort of kid that would ever dart out in traffic or run away.

I am not sure about unsupervised. Our town doesn't have much traffic so it would probably be earlier here than in a city.