You have a few different issues here. Purina is basing that time on the commercial egg laying operations. They manage when their pullets start to lay by using the commercial egg laying hybrids and controlling lights. We don't do that so we don't know when our pullets will actually start laying. That's why you often see the suggestion to wait until you see eggs to switch to Layer. They don't need the extra calcium in Layer to make egg shells until they start to lay.I've been feeding my chicks Purina Start and Grow medicated since hatch. When should I switch them to a different feed? Purina site says to feed till 18 weeks. Then to transition to layer feed. They are 12 weeks and not allowed to free range yet.