I agree totally with Fred. There is no benefit to starting them on Layer early. It will NOT start them laying earlier, no matter how many old wives you listen to tales from.
There are plenty of studies out there that prove the extra calcium in Layer can harm growing chicks. It can damage their bones or internal organs. These studies are usually done on chicks eating Layer from Day 1, so I do not know when is a safe time to switch to Layer. At 16 weeks, yours re still growing, but not all that much. And it is not one bite that will harm them, but a steady diet of Layer. You probably could switch at that age without doing much if any harm. At worst, you would probably just damage their kidneys or liver a little which would make them less able to resist other problems later in life. I seriously doubt you would see any bad results now. If they drop dead a year from now due to their damaged kidneys or liver finally giving out when they are stressed over something else, you will never connect that to them eating Layer early. At 16 weeks, it probably would not harm any of them at all. Probably.
I don't use that brand, but many bags of feed have a chart on them that shows when to switch. The standard is 20 weeks or when they start to lay. I suggest you keep feeding them as you are and get a bag of oyster shell. When you see the first egg, offer oyster shell on the side so the ones that need it will get it and the others won't. Then, when that bag of Starter/Grower runs out, switch to Layer.
