I think it all really depends on location, weather temp.
Here in So. California, Spring, I still wont put out my Bobwhites into my outside brooder until atleast 4weeks. Coturnix variants, at around 2weeks. Basically, when they nicely feathered.
Exactly, I've had mine out as early as 3 weeks, with temps getting down in the 40's. It's not highly recommended. Chances of loosing alot of them are very high.
BUT, mine have a draft free warm "brooder" box to come and go as they please. I'll keep them inside of it, to become acquainted for 3-4 days before opening the small door to the outside. It can be done, yet alot of care must be taken. Bob's will pile on top of each other in a corner at least 40 high, trying to get warm, crushing the ones on the bottom.