Any old time. They are plenty old enough to old their own in the flock. They should have been interacting with the adults weeks ago.
Since they are nearly full size, the tricks we use to integrate small chicks, a safe pen with chick-size openings, you will need to be sure to provide plenty of space and high perches so the youngster can fly up and out of trouble. Obstructions or partitions help break up line of sight and travel patterns to foil bullying.
Food and water needs to be provided in numerous places and even on another level so the youngsters aren't deprived of nourishment and hydration. I've used an old camp table and my youngsters would hop up there to eat and drink.