I agree that MissPrissy's way is certainly the best!!
That said, I DID add some much younger, but it was a little different in that i had 3x more chicks, then adults. I added the chicks in at 5 weeks. And I was absolutely amazed at how well it went. I put them in with it in my mind that I would need to take them out again, but I didnt. There was a little (very little) bit of pecking, and still if a little one pesters a big one they get pecked, but there was NO ganging up like I've experienced before.
It's hilarious watching my adult hens, they walk around with a look on their faces of utter contempt, but they've been SO good about it all.
If you can wait longer, wait as long as you can. But if you need to move them in early, make sure you do so on a day you can moniter them. I was out to the coop every 15 minutes the day I put them in, as I've seen how quick hens can get youngsters down and bloodied. If your hens end up cornering the chicks (or any chick) and pecking away, you'll need to take them out again or else lose them.