Chickens that are older have a great deal of immunity to disease-causing organisms in the environment due to past exposures. Because of this, it’s more probable that it’ll be a carrier of a disease.
Carriers are individual chickens that, although they may not be showing any signs or symptoms of illness, can spread sickness to other chickens around them.
Because their symptoms are so subtle or non-existent, this can enable a carrier to spread illness to other chickens without the chicken keeper knowing that an individual chicken might be the culprit.
Since chickens and younger chickens haven’t yet developed resistance to many diseases, it’s always a risk to introduce flocks of different ages from different sources.