And I third that,I have 20 pullets at point of lay and there is no way I would put them near my "ageing" flock,I have made another run and coop just for them,they shall be in there until at least March next year,over the winter I shall extend their run though for 20 hens I have managed to make a run 100ft x 100ft and they seem to be very happy,just started laying their tiny eggs yesterday!
I only know this from experience and taking the wrong information from someone,I had once put new birds with the old,it was a nightmare,I lost half the flock back then.What some may not realise is that in this example,within the older flock,some had infectious coryza,seemingly within their system,but were not showing symptoms at all until I introduced new birds.Then some were coughing,some with eyes like balloons,it was a nightmare.
Since then,I always have the most strict bio~security in every pen,they all free range and I know not everyone has the space to do so,but they must must must be kept separated in the first instance.