Integrating 8 week old chooks

Hello,
I have a couple of 2 year old hens and would like to expand my flock. The supplier does not have POL hens, just eight week old littlies. The breeder has advised I separate them and I know that I will need to quarantine the new ones but at what age can they be integrated with the others? 
Thanks,
Jane

My feeling too is that you will have to keep them separate until  the chicks are fully grown.  Probably about 14+ weeks.  If you keep the two flocks in sight of each other they can get used to seeing each other and then, when you are ready, you can put the chicks in the adult hen's coop after dark when they are all sleepy.