When you switch the pullets over to the layer is what most do. Technically he should not eat layer as the calcium is not healthy for a bird that is not utilizing it for egg production but they all seem to do okay on it. So approx 6 mo should be fine.
This is one reason I feed only grower or flock raiser, and offer oyster shell on the side. The young chicks and the roos have never touched the oyster shell when I have been in there, over a period of several months.