From what i've read, use to, the reason you couldn't feed baby ducks medicated feed was because of what the medication (more or less poison) was in the chicken feed and same went for the older ducks. but they have changed what the medication is now and i have read conflicting posts on wether or not to feed it. Best thing to do would be to check out what your feed store has and do some research on the product and see if that particular medication in the feed is safe for ducks by looking it up here/online or even asking your local vet, They can be very helpful on this stuff.
Edited to say: When i first started raising ducks i fed the ducklings medicated chick starter and they turned out fine, but its up to you.