Ours were born October 26th and they're outside in the coop by themselves (no mama hen) and without a heat lamp or any heat source and they are doing great.   It got down near freezing last night and this morning they were still right as rain, active little engines. Their coop is well ventilated though, and well sealed in all areas other than the ventilation doors.  They're way more active out there in the cold and seem 100% happier.  They are Norwegian Jaerhons.
 
But, I kept them in a room inside our house that was set at 60 degrees from week 3 onward with zero heatlamp and no heater.  The week and a halfers we have in that same 60 degree room have a heatlamp that I actually had to move because they were lethargic and too hot.  They are doing great at 70 degrees (with the heat lamp further back) at 1 and a half weeks old.  Those are easter eggers, brahmas and a golden buff.  They have each other to snuggle with when they get too cold, or they can move closer to the heatlamp end (they prefer to use  each other).
 
I have a Silkie (just one) who doesn't like it any cooler than 85 (she's a fragile little thing), so she's in a different brooder along with an easter egger, and she hangs out under the lamp all day while the EE spends her time on the cool end.
 
I'm not saying all the experts who tell you that a brooder should be at 95 and decrease by 5 degrees every week are wrong, because I'm the newbie, not them and I know very LITTLE about chickens.  But...ours are doing great in cooler temps and didn't do very well at all when I followed the instructions.
 
The best advice I ever received from anyone here at BYC with regard to raising chicks was "Let the chicks tell you what temp they need, not some book."  Best advice ever.  Chickens are individual, unique lives.  They will huddle and shiver when they are too cold.  They will spread their wings out as they lay down, and open and shut their beaks and just act "blah" when they are too hot.  Watch your chicks and you'll learn really fast what temps they are most comfortable at. 
 
The strong will do well even at temps cooler than what everyone else says they should be at.  I want strong chicks, so I reduce heat early and make them adapt.