The best time, is when they are the same size as the older hens, or nearly the same size (this will allow them to stand up for themselves and be less likely to get pecked to death).
If I were you, I would start to let them get use to seeing each other. Keep the older ones where they are, but put a small pen (a removable one, dog pen, or anything else easy) next to the older hens, and allow the 7 week olds to run around next to the older girls.....but at the same time, they are separated and out of harms way.
This will allow you to watch and monitor how they react to each other.
Once you see that they are getting along, you can do supervised time with all of them together.
Good Luck and Best Wishes,
Laura