I have hens of various ages, some are 13 weeks old and some 11 weeks. I know the average age to start laying eggs is 20 weeks...will my girls start laying this fall? I also wonder if anyone knows how many eggs I would get a day with proper lighting from 12 hens? Also, I live in Ontario (Canada) and it gets cold in the winter. Will they lay in the winter?
Does anyone know how to keep their water unfrozen in the winter?

There are water heaters you can buy to keep your water from freezing. They can be a fire hazard though so you have to be careful. Here I just place their water in the sun during the winter and we do okay. (Doesn't get below freezing often here.)
Your hens should definitely start laying this fall. (Most will start around 4 or 5 months.) In the cold weather they'll slow down production, but they'll still lay. In good weather and light, they should lay maybe 5 or 6 eggs a week, sometimes 7.

Hi Heidi, down here in the mid-atlantic region of the states, my 7 hens hardly slowed down over the winter. They hatched in March of 2000 and started laying about August. When winter came it came fast, had the coldest November and December in almost a century, or so the weathermen said. They slowed a little then, but I still got at the very least 3, usually 4 eggs a day, all winter long. I didn't use lights to extend their days either. Your birds will be stressed more trying to stay warm up there in Ontario, so you may not get as many eggs. Maybe someone much closer north can better answer your question. Keep in mind that eggs are mostly water, so your birds will definetly need access to clean, unfrozen water this winter. Good luck!

Thanks for the response you guys! I am wondering if you keep any roosters with your hens? Presently I have 3, some of which I won't be able to keep.

Here in the UP of Michigan we insulate our chicken houses and use the timer for light during about eight months of the year. Right now the timer is off, but it will be back in use in a month or so. I prefer to keep only one rooster with the laying hens because more than one rooster usually leads to problems when they are confined in the winter. If you have good dual purpose hens, you should expect that they would lay 6-10 eggs per day during the winter if their feed, water, comfort, and stress are all at optimal levels. Good luck!