Thanks,There is value to keeping the older girls around; they share knowledge and "culture" with the new ones coming up, and the fewer disruptions there are to the pecking order, the better. Every time a bird is added or removed, there is tension, so I would only remove if you're really limited in resources and can't add birds without culling.
I've got 7 young ones coming up. Not sure how many roos and pullets but they're all staying. I'll just add them to the flock.