I don't know. There have been plenty of studies that clearly show feeding Layer feed will harm a chick. It's the extra calcium in the Layer. Studies clearly show that chick mortality rates are higher and when they cut the ones that did not die open to examine them, they see damaging growths on internal organs, especially kidneys and liver. They can also get bone deformation.
The problem I have answering your question is all the studies I've seen start out from Day 1 feeding the chicks Layer. I have not seen any studies that have the chick on Starter or Starter and then Grower for 4, 8, or 12 weeks before they switch to Layer. I can't tell you the absolute minimum age it is safe to make that switch.
I think it is also important to note that it is not the percentage in the feed that is important but the total amount of calcium they eat in a day that is. If they only eat Layer for ten minutes a day and the rest of their food is from free ranging, then the Layer is not likely to hurt them at all. Also, the damage is not going to come from eating one bite of Layer. The damage is a cumulative effect from eating the Layer over time.
I find it real easy to feed all of them Grower with oyster shell offered on the side until they are all of laying age.
Good luck!!!