Taking in older birds is not as simple as bringing them home and putting them in with your current birds. There are a few things to consider for a safe and successful introduction.
1. What breed(s) are they and will they get along with your current chickens? I've read that Polish are often picked on by other birds because of their head feathers. And some breeds, such as hybrid production layers, tend to have a short lifespan, many only living to about 3 years.
2. Where are they coming from? Have they been well cared for and were they kept in a clean and spacious environment? Do they show any signs of disease (respiratory infections, feather problems, bugs, lethargy, poor comb color, weight loss, etc.)? If so, do not take them!
3. Are you prepared to quarantine then introduce them to your current chickens slowly? A strict quarantine is advised when introducing adult birds to a flock. This means they must be kept far away from your current birds and you must shower and wear completely different clothes and shoes when going between the new birds and the old birds for an entire month. After the quarantine and you have confirmed the new birds look healthy, you should introduce them to the old birds slowly by first keeping them in separate pens and letting them see but not touch each other for at least a few days.