Day old chicks from the hatcheries are about as safe as you can get. Not because they are day olds, but because they don't come in contact with the other chickens. There is risk in anything you do, but it is extremely rarely that you see a problem with day old chicks from a major hatchery.
If they are a month old, they can be carrying some diseases, even if they have not been directly exposed to any other chickens. Some chicken diseases are airborne and some can be picked up from the soil or certain creepy crawlies they might eat. I would treat month old chicks the same as any other adult chicken.
It is possible that they have picked up a disease and developed an immunity to it. Coccidiosis is a great example, but there are others. If they are infected yet have developed an immunity, it does not matter how long you quarantine them. They will never show the symptoms. One way to test for this is to pick an expendable chicken from your existing flock and expose that chicken to them, but also keep it quarantined from the rest of your flock. Don''t forget, it is also possible your existing flock is infected with something that can infect the newcomers. You never really know.
Many people mix new chickens with their flocks all the time and few actually have problems. But when a problem does develop, it can be catastrophic. I think quarantining those chicks would be a very good idea. If you can manage to also include exposure to an expendable chicken, even better. But that is just my personal opinion.
Good luck however you decide.
