Chickens are very photosensitive, and many need a minimum of 14-16 hours of light a day to lay well. More than 18 hours will actually slow down egg production, and light 24/7 will cause lots of stress and possibly cannibalism. All animals must have a dark period each day for their bodies to function properly. http://umaine.edu/publications/2227e/ is a good discussion on poultry lighting for all stages of a bird's life.
I have not seen any research that shows that the chickens' life span will be affected by supplementary lighting, although here is an interview with an avian vet that says it does not: http://www.the-chicken-chick.com/2011/09/supplemental-light-in-coop-why-how.html.
There is a possibility (I've not seen any research one way or the other) that the hens will stop laying at an earlier age if they have lighting in the winter. It seems like it is basically a choice between getting most of your eggs in the first 2-3 years or spreading that number of eggs out over more years. Hens, like humans, have a finite number of ova and so you can't make a hen lay more eggs than her body is hatched with. For those of us who keep chickens for egg production (not just as "pets with benefits") and replace them at 2-3 years, it ABSOLUTELY makes sense to use supplementary lighting.
I have some broody hens that are going on five years old, and they still look great even though they have supplemental lighting every winter. I have no idea how well they lay, as there's no way with my flock to really know who lays what, when, but that's not why I keep them around, anyway.
Anyway, whether or not you provide lighting in the winter seems to be a personal choice, dictated by economics (you willing to buy eggs all winter while still feeding hens?) and personal choices about the hens' lives (do you feel like the hens need a break every winter, for longer than their natural moult period?). My hens moult every fall, and get their rest then. As soon as I see them moulting, I turn on the lights for a bit longer every week until we're up to 15 hours of light. Most of my flock lays right through the winter, which pays for their feed. For me, there is no other choice. You'll have to make up your own mind.
Thank you for this info! Clarifies a lot of questions for me. Outside of breeders I don't plan on keeping my flock off the dinner table longer than 1-2 years anyway, but I didn't get them to be pets. Solar seems to be the way to go for me since I don't have electricity at the coop - and can't afford to run it the 300' or so from the house. Can't see it being cost effective to spend that much money running power to the coop when the only thing it's needed for is winter lighting. Maybe if we had more severe winters it would make sense, but not for my location. This year I'll probably just do the solar string lights, but next year I want a system in place that I can set to certain times (on a couple hours after dusk and again a couple hours before dawn). We had an awesome lighting system for our saltwater aquarium that was set to several stages of lighting (dawn-daylight-dusk-moonlight). I'd like to do that for the chickens if I can get it solar powered. If I remember right though, that lighting setup was 300-600 and the bulbs alone are $60 a piece. I'm hoping in the drier environment of the coop vs. fish tank it will last a bit longer.