I usually let mine go together at about four weeks. The momma hens will guard the chicks. My opinion is that the younger chickens don't have a problem with the chicks. I usually sit with them the first couple of times until I see that the other chickens aren't paying much attention to the little ones. What I noticed is that my flock learns from their experiences and it gets better and easier each time.
I usually let them go together late in the afternoon/evening about an hour before they go to roost or however much time I have to spend with them. I do that a couple of days and see how it goes. Then maybe I'll let them together for a few hours later in the evening before roost. I go by when the sun sets. That way the chickens are tired and not hungry. One hour before sunset is when they start going to roost. I start with that first hour before bedtime and then add more time each day.
Edited to add:
I start at the 4 week mark because my broody hens start at anywhere from 5 to 6 weeks (usually) to leave the chicks on their own. This gives the chicks time to be with the flock while the momma can guard them.
Thanks for the reply, I think I'll give it till the ninth when the youngest chick will be four weeks then I'll try letting them out and watch how it goes. For now I'll start advertising for a home for another one of my paints that is turning out to be a boy

If he goes quick I'll only have two juveniles to worry about with the chicks.