They need to be fully feathered, in a place that is safe (forgive me for raising my "voice") SAFE from predators, heat, cold, toxic materials. Some feel they are fine outdoors when fairly small. In some cases that is true, but I suspect in those cases there is already a very sturdy, safe well-equipped shed or barn involved.
Because this question is so very often asked by people who are having a hard time managing the ducks' water, may I ask if cleaning the brooder is becoming a problem? How old are they?
If the answer to the first question is yes, take a look at 70%cocoa and tweetysvoice brooder and watering station.
Please Please do not put the ducklings out hastily because of indoor cleanup problems.