For me the cold isn't the issue. It's the damn short daylight from Nov to March-ish that's the problem. I'll do a night ride once a week and get out during the day on weekends. Night rides are fun but I'd still prefer day light.
I would say you can ride all year long, but it gets to the 20's and 30's in the winter, so it depends on your tolerance with cold.
too much for me and the few people i ride with.
Down here, we don't ride muddy or wet trails, so even though we ride year round, if we get a good day of rain, we can't ride for at least a week.
Weather is one thing, I also wanted to make sure that the trails were actually "open" during the cold weather. It's nice to know that the trails are generally rideable year round.