This is just my experience and it may not work out the same for you.
I had some that were just 3 weeks apart and although they never roosted together they seemed not to bother each other while free-ranging. In fact the 6 week olds were sometimes too jumpy for the 9 week olds who had begun to mellow out from their wild days as toddlers. Every once and a while a young roo (6 weeks) would challenge the 9 week old roo and the older guy would peck the younger one and the lesson was learned. It didn't happen too much after that.
I must let you know that the 9 weeks were larger breeds and the 6 week olds were bantams and I have a small flock (2 adults, 2 nine-weekers and 15 six-weeker bantams). Plus this happened while they could free-range and the babies could escape by running away.
If your situation is like mine then I would watch to see how they do. If you are talking about a small enclosed pen then make sure the babies will have somewhere to run and hide. Either way it doesn't hurt to try. You could watch for a bit and if it starts looking ugly -- separate them again.