I would say once they're fully feathered they'll be safe to go outside. At this time of the year it wouldn't be bad to have the option to bring them back inside if the weather where you are turns inclement.
I agree, and just wanted to add that it also depends on what they are used to. If they are under heat lamps now and it's cool outside I would proceed with caution. I had some 2 month old poults get caught out in a blizzard and they all got separated, but they were raised outside by their mother from day one so they were more prepared than poults raised in a brooder under heat lamps. They all survived but I was worried for a few days until I got them all back...
I brooded my turkies in a 4x4 "coop" outside. Let them range around a mini yard at 3 weeks or so. It was april and we got dusting of snow now and again. Just acclimate them and provide heat if it's too cool.