I agree with ChickCrazed on the topic, but you also have to be aware of exceptions to a hard and fast no heating rule such as their age, a point which you bring up, LK. I've actually been working on a neutral-stance article on the topic that lists reasons why it's unnecessary to heat the coop under normal circumstances, a few reasons why, some risks, some safety tips, and exceptions to adult chickens not requiring heat. Here's an excerpt from what I have thus far, strictly a work in progress but a lot of research has gone into it and this part is what particularly interests me:
It’s a common misconception that because we feel cold in wintertime, our chickens do as well. The fact is, birds like chickens are very different than mammals like us, and chickens have excellent adaptations to handle the cold!
Behavioral Adaptations to the Cold
What can be noticed the most is behavioral adaptations. You may see your chickens huddling together with their feathers fluffed out for some period of time, holding one foot up in their feathers, even tucking their faces in under their feathers when the temperature is particularly low. These are not causes to be alarmed, but natural behaviors to help the birds keep warm!
What behaviors are
not considered normal? If a bird is stumbling when it walks or unable to maintain its balance, it likely needs help. Other signs that might be significant are if a bird is sleeping a lot more than the others, or generally not acting right. What is best for the flock, unless you have a very small flock anyway, is to have another pen ready to heat for individuals that struggle, lowering the overall risk to the flock.
Physical Adaptations to the Cold
In the cold, most animals can restrict the blood flow to their extremities to prevent losing heat in their core. By doing so, however, they risk severe frostbite in those extremities and it can result in some uncomfortable conditions. Ever come in from working in the snow and have your fingers swell painfully? This is essentially the result of this process.
For birds, however, this works a little differently. Though they, too, can restrict blood flow in order to prevent heat loss, they take this a step further. The veins and arteries in a bird’s feet are wrapped closely together, allowing for what is called countercurrent heat exchange. As the cold blood moves through the bird’s veins back toward its core, and the warm blood leaves the bird’s core toward its feet, the veins and arteries exchange heat. Basically, the warm blood warms up the cold blood before it goes back into the bird’s core and the cold blood cools down the warm blood as they pass, thus lowering the amount of heat lost to the air through the bird’s feet.
Perhaps the most remarkable of birds’ adaptations, though, are those unique and ingenious feathers! Feathers come in many shapes and sizes, perfectly adapted for many habitats across the planet, and even a single species of bird has many different kinds of feathers on it! For the purposes of this page, I will focus mainly on the types of feathers that matter most in wintertime: contour, semiplume, and down feathers.
Most feathers consist of at least two of three parts; all feathers have a shaft, and most have either a vane, down, or both. The central shaft is what gives the feather its shape and attaches it to the bird. The vane is the flat, smooth section of the feather toward the top, and the down is the fluffy, loose part at the bottom of the feather. Down feathers, as you probably are aware, consist entirely of down with a soft and flexible shaft. Contour feathers have a vane and some down on a stiffer shaft, and semiplumes are somewhere in the middle, with more down and less of a vane that is not held together as smoothly compared to contour feathers.
How these feathers help keep a bird warm is simple. The vane of the contour feathers forms a tight, sealed barrier from the outside air, restricting the heat the bird produces from leaving its body. The downy section of all of these feathers then traps that heat beneath that barrier, and holds it close to the bird’s body. By fluffing out their feathers, chickens produce an even thicker layer of insulation—and as long as the vane barrier is uninterrupted, that heat can be held onto to easily maintain the bird’s body temperature.
Feathers are such excellent insulators that you may notice that the vane barrier of your birds’ feathers feels cool to the touch in cold weather. Want to know how cold your chickens really are, though? Lift up your bird’s feathers and put your fingers down into their down near their skin. It’s quite warm in there, isn’t it? This is also the reason why chickens struggle more in extreme heat than in cold. It’s easy to keep
in the warmth they produce, but releasing it is more difficult!
Acclimation to the Cold
When providing heat to your birds, you are robbing them of the opportunity to acclimate to the cold. This may not seem like a big deal, but in many areas, the power is often knocked out over the winter and it leaves these birds unprepared for the sudden change in temperature.
This comes with an additional concern, in that your birds will not want to leave their coop for any length of time if they are used to the supplemental heat. This raises the risk of boredom amongst your birds, buildup of excess moisture from extra droppings and the birds breathing, and lowered activity and exercise.
Many, many people also state that by providing heat, you are preventing your birds from preparing themselves for the cold in another way. The theory is that chickens will grow more down when exposed to cold weather, thereby preparing themselves for what is to come. If they are in a heated coop, they will not have that pressure to grow extra down, and so will be even less prepared for power outages and temperature changes.
As with everything, there are exceptions to strictly avoiding heat in the coop.
Vulnerable Breeds
Certain breeds are known for being less cold-hardy for one reason or another. Mostly, people seem to agree that breeds with large single combs are most vulnerable to the cold, although this has not been my personal experience. What breeds have been shown to be most at risk in my experience have been bantam breeds.
Bantams tend to be less hardy in cold conditions simply because they are producing less heat in relation to their surface area than a large fowl bird; they have less thermal mass. Now, many, like bantam Faverolles and bantam Ameraucanas, are perfectly adapted to the cold and shouldn’t need help. Some bantams, however, are thinner and less fluffy, and as such they have less insulation from the cold. Most will adapt just fine, but it is advised that you keep a close eye on bantams in the coldest temperatures.
Feather Shapes
Though in a lot of cases, I must disagree with the consensus that a particular breed is not cold-tolerant, there are certain breeds that beyond a doubt are at risk in cold weather. Mainly, this is because of their feather structure.
As you are likely aware, there are certain breeds that have funny feathers. While in most breeds, the feathers are absolutely perfect to hold in heat and insulate against the cold, certain breeds have feathers designed a bit differently. Frizzles, with their curled feathers, are less able to hold in heat as a result of lacking that solid feather barrier as was explained above. Hardfeather breeds like game bantams may have difficulty in the cold as well because they have harder, less downy feathers that can’t insulate as well.
And Silkies? Well, Silkies and other silkie-feathered breeds are lacking the barbules that hold the vane of the feather together, producing an overall downy appearance. Whether this makes them less able to handle the cold is debated. Many people insist that silkie feathers don’t hold heat, while others say they hold heat better than regular feathers. Logically, by looking at the structure of the feather as was described above, it can be assumed that Silkies would have a harder time holding in heat because they lack the solid part of the feather that forms a barrier between them and the cold. Personally, though, as the owner of one Silkie who took the cold in her stride and didn’t bat an eye when we dropped beneath 0, my experience has been that they can handle the cold just fine.
This all is mainly focused on the adult bird aspect of it. As for your chicks, ChickCrazed hit the nail on the head--Observation is key in knowing when or if they will be okay without heat. Once they are fully feathered and weaned off of the heat lamp, then keep a close eye on them and use your best judgement. 
ETA: Okay, rereading this... I thought there was at least something about chick down in one of the above sections to tie it in with the subject at hand, but I guess not. 
Anyway, I feel it is important for people to know the above in order to make a fully informed decision on the heating versus not heating debate, so I guess posting it works out somehow. 
ETA 2: Further research has led me to adjust a bit of the information under the Behavioral and Physical Adaptation sections.
ETA 3: And to be clear--the above info applies to chickens mainly and may not apply fully to other species! Things like feather structure and countercurrent heat exchange are true for all bird species, however!
ETA 4: Because what's one more, right? I'm also working on a section for extreme temps and a few other things for this, so any input or critique is appreciated!
dont get me started on that!! I have a job interview Tuesday, Wish me luck with that, really need a 2nd income around here. The SS still has no job he isn't even looking. A friend of his wanted to get him on with the Laborers Union but he just disappeared for a few days and messed that up!!! Getting real annoyed at him. He is 23 and still running around acting like he is 16. Wanting money for this and tghat and not doing snot around here to help out even. I paid him 100.00 to help me get the barn and the garage cleaned out for the winter,,,, NOT!!!! I didnt want to give him the money till the job was done but DH said he would make sure he did the job!!! Well it's been 3 mths!!! End rant. just had to get that off my chest, thanks. ok it's 4 am and I have an hour before I have to get up and start my day, going to bed, the dogs are in and I'm tired, G-night all
