definetly at age 6 months. all baby's start out with a blue hue to their eyes at birth because of the oxygen they got in the womb, and it takes time for them to well, adjust to liting etc. that causes them to change if they are going to.
They are not "set" according to our doctor until 3-4 years old. Usually around 6-12 months you can tell if they are going to be blue or brown, but they can still change hue in that color a lot until they are about 3-4 years old.