Honestly, I can't remember what the exact age they need to be before they can go out. I know when by looking at them when it is time. I'll tell you what I know though. Whether you are introducing or not chicks have to have their feathers before they can really go outside so that is number one. Slowly lower the temps of the brooder until they are used to the temp outside.
They have to be integrated over time cause the older chickens will harass them. So you should segregate them away from the older ones but within sight so that they can get used to each other. Then after a week or so you could try to let them out together under supervision to see how they act together. There may be some chasing but that is them establishing the pecking order. Just don't allow them to attack and draw blood. If this happens, remove them. Also, you can move the new chickens in with the older chickens at night. They will sometimes wake up in the morning and never realize there are more chickens. They still need to be within sight before this method also though. I hope this helps a little.