Had one group of chicks that roosted on the main roost at 6 weeks.
Was at the far end, but still... I was flabbergasted.
My first group of integrated chicks were allowed on the main (top) roost at 5 1/2 weeks, at the far end of course. Only had 3 hens at the time which helped.
My current group of chicks began roosting at 4 week 1 day. They would love to sleep on the top roost, but the hens won't have it, so they're on the bottom roost for now.