I have a baby chick that will be 1 week old tomarrow and 3 that will be 2 weeks old 2 days later on sunday?

the 5 days older chicks are in there own pen with the brood hen and the younger single chick is in the house being the only hatchling from the eggs i took from the hen because she was more concerned with them than the already hatched and now im wondering when i should introduce the single chick to the older babys and mom and later to the rest of the adult flock? any other advice is welcome.