At about 4 weeks I start giving my chicks grass, vegetable scraps, rice, yogurt, etc. I find it funny to watch them squabble over 2 or 3 pieces when there is 10 more laying there. My ducks are about 1 week old but I can't see why they would eat any differently.
heck after 2 weeks we put ours outside in a screened in tent we got from wal-mart for $35.00 now its $48.95 works well and when it rains they love it. http://www.walmart.com/catalog/product.do?product_id=5493341 The wife has been giving them lettus cut up in water and they love it. when we clean the pen out they go out and start eating grass. so it depends on them. If they don't want something they won't eat it.