I have 4-6 wk old chicks and 1 ended up being a rooster so now I will only have 3 hens. 2 Wyandotte and 1 Orpington. Would it be ok to add an older hen to the flock? If so, what age would be appropriate?
You will probably want to wait to introduce her until they are about a month older. Some hens are motherly and won't beat up on the chicks, but others will make it known that they are flock leaders and can really hurt chicks.
When introducing her, you will want to have them separated for at least a week. I isolate all new chickens at least a month. Sometimes 2 months. Depending on the source.