I take my pekins out when their 4-5 weeks just depending on the ducks themselves and the weather outside. I also keep a lamp out there for them thats on at night until 6 weeks or so. As long as they have a safe place out of the weather an wind their fine.
my 4 week old ducks are outside, no heat lamp, but a snug, insulated dog house with latching door, and tons of shavings... last night it was 33... they are fine!
Mine are 3 weeks old and I kick them out all day as long as the temps are above 60. Swimming outside when it goes above 65. Swimming is still in a plastic tub that is not very deep, they can stand in it with water to the top. Deep water swimming is still only supervised. They love to drive, I just want to make sure they can get out, so the shallow one when I'm not able to watch. At night they are still sleeping in the brooder box in the kitchen (still freezing temps at night). Ironically they are so dead in the evening, that they sleep most of the night and do not make such a mess with the water and food bowl.
Katharina