I recommend waiting until the younger ones are around 4 months old if at all possible.
I agree with ddawn. They should be close to the same size to be fair to the younger ones. If you are just not set up for two flocks, you can use a dog crate to keep one of the groups in inside the coop or garage at night to keep them separate (after brooding is over of course and they are off heat lamps).
During the day, you can fix a temporary pen with netting overhead, but be aware that this kind of pen tends not to be predator proof and thus dogs might get your flock unless it is sturdy.
I have used a dog kennel as a growout pen (but you must modify it for safety).