I did start w/ medicated because that is what the clerk sold me when I bought the 1st bag.  I have read that either is fine, but the medicated helps the birds build up a tolerance to a normal GI bacteria (cocci) that is present in all chickens.  But like E. Coli in people, if there is too much of the bacteria it can sicken and kill the chicks.  Older birds have better resistance to the problem, but they developed the resistance as they grew.  If older birds are near to young chicks, they can introduce high levels of cocci and this is BAD for the young ones!  Being outdoors on dirt can also help build their resistance to the effects of cocci.