At what age do I mix in the new ones

[rogerf]

The young ones (10 qty) are now about 12 weeks old and are living in a temporary coop attached to the same run as the three older chickens.
I have kept them divided but recently started to mix them but the older ones as expected are going after the younger ones and sometimes actually keeping them held hostage inside their respective coop. Am I starting too soon or too late.
I plan to enlarge the original coop to hold all of them since they will all be laying sometime soon and they will have no choice but to be all together.
Any help is appreciated

 

[Ridgerunner]

This will give you better information that I ever could.

Buff Hooligan’s Adding to your flock
https://www.backyardchickens.com/web/viewblog.php?id=2593-adding-to-your-flock