Medicated feed is safe to feed for a few weeks at any age. The medication is actually a thiamine blocker, not an antibiotic. It only help protect against the coccidia protozoa. It will not protect your chicks from bacterial or viral infections. The amount of 'medication' in the feed will not stop an active outbreak of coccidiosis. For that, you need a more concentrated form of the medication. If you 2-week old chicks haven't been outdoors yet, there is little chance that they have coccidia. It's actually a good idea to get them on the medicated feed before you start transitioning them outdoors. That way, they will be able to develop resistance to the protozoa in the outside environment without their systems getting overwhelmed.