Follow the directions on the bag. Different Manufactures recommend different times depending on their food. Usually they're on starter until they're around 8 weeks, then a finisher intil 16 weeks then to adult food. But check your feed bag to see what it recommends
There are plenty of studies that show that high calcium, like they have in Layer, can harm a growing chick. So don't feed Layer until they are laying age or start laying. 20 weeks is a standard age given, but there is nothing really magic about 20 weeks. If they start laying earlier, you can switch. If they are not laying at 20 weeks, you can wait.
Layer should have somewhere between 3.5% and 4.5% calcium. You can find it on the analysis label.
Most other feeds will have somewhere around 1% calcium. Any of those will do until you switch to Layer. You can also just continue to feed these and offer oyster shell on the side. The ones that need the extra calcium for eggs will eat it and the rest should not eat very much of it.