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Generally - and it's a big "generally" - chicks don't need to be quarantined because they aren't old enough to have developed immunities to something from whence they came, to be carriers of anything dangerous. Also, if you get them from the same feed store (for example) you are getting them probably from the same hatchery, from where the feed store purchases them.
Juveniles need to be quarantined from adult birds because they also have not developed good immunity systems against whatever YOUR chickens may have developed immunities against. This doesn't apply to broody-raised chicks, by the way, as they pick up immunities from the hen who hatched them.
Newly acquired adult birds need to be quarantined from your flock because they HAVE developed immunities from another location which might not be compatible with those of your flock.
Kind of a simplistic explanation.... someone is sure to comment with corrections, additions, etc.
