It's best to wait until they start laying. The basic difference between all the feeds except Layer is the percentage protein, and ti be honest, it does not matter that much if it is 15% protein or 20%. They will do fine on any of it at about any age.
The difference in Layer and the rest is Calcium. Hens that are laying need extra calcium for the egg shells. Chickens that are not laying don't need it. Their internal organs have to work extra hard to get rid of the excess.
You can wait until they start to lay then switch to Layer or you can just offer oyster shell on the side and keep feeding whatever you are feeding them, at least until that bag of feed runs out. Some people never switch to Layer, just depend on the oyster shell for the extra calcium.