One of the first things I do each morning before going to work is to check weather.com for my zip code. If the temperature indicated is below 20 degrees, there's a good chance it won't be a riding day for me because my bike probably won't start. If it's below 10 degrees, then whether it will start or not isn't an issue because I'm not going to try it.
There is such a thing as too cold to ride - for my commute to work each day it's about 15 degrees. At that temperature and at 60 miles per hour I've found that even behind the windscreen on my Electra glide, my face begins to freeze in the short 20 minutes I'm on the road. It can take several hours for my cheeks to stop hurting. I could switch to a full face helmet, or wear a mask, but that would interfere with my morning cigar. So for me, 15 degrees is my limit.
Most important to winter riding is to know YOUR personal limit. As mentioned above, I'm good down to about 15 degrees with the gear I currently use, on the bike I currently ride in it's current configuration, and on my regular ride to work. I know I'm not going to be on the road more than 20 minutes. I also know the physical condition I will be in when I arrive. I know this because I make the ride nearly every day and have progressively approached that limit.
My limit would not be 15 degrees if I traveled further because I know hyperthermia and/or frostbite would quickly set in. I know I could withstand a little colder temperature if the distance were less. By the same token, I could ride a little further if the temperature were higher. In either case I would need to find a new limit by a progressive approach.
A word of caution: If you've never ridden below 60 degrees, do not assume you can ride in 15 degree weather for 20 minutes! To ride safely, you must learn your own limit by gradually expanding the window of what you consider good riding weather. If, however, you've never ridden in weather below 60 degrees there's a chance you're missing a significant part of the riding season. Start slow and work up (or would it be down?) to it. You'll soon find there's a lot more good riding days than you ever imagined.