Chickens are not typically fully feathered until at least 6-8 weeks old. They may appear feathered sooner, however if you look them over closely there are still downy spots under those wings which need to fill in before exposing them to temps below 70.
They need all feathers to maintain their own body temps, but also remember that they need time to acclimate to severe drops in temperatures. You are putting them at risk of stress and health conditions if you throw a 6-8 week old chook out in 40 degree temps without proper acclimation time.
The rule of thumb is by 6 weeks, they should be able to handle approx 70 degrees. So if your lows are around 70 at that time and you put them out they should do fine. Then as temps start to gradually decline, they are aging and fluctuating their own body temps to match the environment.
Now there are exceptions, especially in warmer climates where they do not need to worry as much about drastic changes in outdoor temps. But I know for up here we can go from 70 degrees to 40 degrees in a single day...sometimes less than a day. What is most important is timing and having a backup plan if things don't go as planned. Be prepared to monitor your flock, especially youngsters during season changes (and at all times for that matter).
We hatch year round here as well, but we also have nurseries set up with electricity and heat lamps to introduce youngsters to the outdoors during fall/winter if necessary. They are gradually weaned off of it by regularly increasing the height of the heat source to allow them increasingly lower temps.
I agree on the statement if you live in a cold climate it is not good to regularly heat your adult coops. Chickens can acclimate well to your environment if you do it right. Offering extra heat is not wise and defeats the goal of a strong thriving flock. Not to mention if you have a power outage and they get stressed by the cold because they are not used to it.
Not everyone is set up to handle winter hatching or raising babies during winter. It is always best to do what is right for your particular situation, regardless of what works for others. Know what you can handle and determine if you have space to move them back inside if necessary (this is especially important for freezing climates). If you can't, then it's probably not a good idea to hatch late in the season.
Jody