I started taking the first group outside at a week old, for about 15-20 mins or longer if they were pecking around, trying to fly etc. Same with the second group. By the time they first group was 4 weeks old, they were fully feathered, and staying outside in a chain link dog pen for most of the day. They were absolutely fine with it and enjoyed it. They had two insulated dog houses filled with hay to go if they wanted. I brought them in when the sun started going down and the air was cooler. Obviously there were NOT in a draft free area.
Around that time, we decided to move them into the coop (10'x20' building with a 20'x30' run. The building is NOT insulated, and has spaces between the boards like a barn does.) with a 100 watt bulb for heat. (BTW, they did NOT use the light, even at 4 weeks old!! Their choice!!!) We also build at 4'x8' "brooder" on top of a table in the coop. The sides were plywood, and in essence, a huge wooden box for the 3 week olds. They had a red heat lamp, and enjoyed cuddling under it at night but during the day, they stayed on the opposite end of the box. Within a week of being in the brooder, I went in to feed one morning, and all chicks had flown out of the brooder, and were in the coop with the 1st group. I let them all out int the run that morning, left the pop door open so the little ones could come back in and get under the light if needed. I moved a heat lamp into the coop area and low enough to the floor for the wee ones. (this group has been much slower to feather on average. Some fully feathered by 5 weeks, and others looking like 3 week olds with lots of fuzz still) I put the babes back in the brooder that night.
The next morning, they were out with the others again. Now I know I could have covered the brooder, but since everyone was getting along, everyone was content, and warm enough, I decided to let them stay out with the week older gang.
Then we got 6 chicks who were a week old. They didn't stay in the house at all. STraight into the brooder outside with a heat lamp at night, but not during the day as it was in the high 80's, low 90's by then! At night it would drop down, so they needed the heat. One day it was raining and I left the light on all day. At 2.5 weeks old, this group decided to fly the coop! Went in to check on them, never imaging they could have flown 4 ft high! But they did, and were out in the coop, but staying together. So from 2.5 weeks, they have the option of being outside all day, or staying in the coop.
This past weekend, the temps were in the 90's and ALL the chicks were laying in the coop, wings spread wide, and PANTING! Forget about draft free!! I put a fan in the gable end of the coop to pull the hot air out, and one box fan in the floor to cool them down. I ended up adding a second box fan! Put ice in the water, and fed them all COLD soured milk with their starter mash. They gobbled every crumb up, and finally cooled off and were comfortable. Those evenings they finally went into the run when it was shaded. Needless to say, they sure didn't need the heat lamp at all those nights, but I did turn the box fans off, and just left the exhaust fan on.
All that to say, go with your gut and how the chicks are acting. If they are cold or too hot, you'll know, and can deal with it. I don't believe there is a "right and wrong way", because there are always different circumstances.
I currently have 31 chicks, and they are all locked in the coop at night.