Hello 
and Welcome To BYC! If you haven't seen it, there is a nice article in the Learning Center on combining flocks https://www.backyardchickens.com/a/adding-to-your-flock Depending on where the older chicks are coming from you might also want to consider doing a health quarantine. Otherwise that is enough of an age difference where you might want to do the gradual introduction where the two flocks are able to see each other but no touching for awhile, through wire seem to work best, ie dividing the coop into two sections or keeping the new/younger ones in a cage inside the coup for a couple of weeks. The chickens will get to know each other and sort of work out a pecking order before actually coming in contact with each other. After a week or two, letting them free range together is a good idea and should help... It will take a couple of weeks to get the pecking order sorted out.
and Welcome To BYC! If you haven't seen it, there is a nice article in the Learning Center on combining flocks https://www.backyardchickens.com/a/adding-to-your-flock Depending on where the older chicks are coming from you might also want to consider doing a health quarantine. Otherwise that is enough of an age difference where you might want to do the gradual introduction where the two flocks are able to see each other but no touching for awhile, through wire seem to work best, ie dividing the coop into two sections or keeping the new/younger ones in a cage inside the coup for a couple of weeks. The chickens will get to know each other and sort of work out a pecking order before actually coming in contact with each other. After a week or two, letting them free range together is a good idea and should help... It will take a couple of weeks to get the pecking order sorted out.
