When I started bike commuting seriously at 30 years old I'd ride down to 5F. Then over the next 22 years my low temperature threshold rose to 21F. Three years ago, after a snowy winter kept me off the bike and on the treadmill more than I wanted, I decided to push my threshhold down to 12F. Why 12F? and not 10F? That was the temperature the day I decided to ride, temperture be damned. Two winters ago I got a new bike and bought studded snow tires for the old commuter and symbolically pushed my low temp threshhold to 10F.
As far as how to dress, well, they are always discussing this in the winter cycling forum here on bikeforums. I found I had been dressing too warmly all these years and perspiring too much, and that would freeze and keep me cold. That if you can stand being a little colder for the first 10-15 minutes, the rest of the ride is pretty comfortable and much drier, owing to less sweat; and moisture is the enemy.
Every person is different, but the one thing that seems to hold true for everyone is to cover exposed skin at some point. Then it is keeping track of what you wore and how it worked out. I commute with a backpack and always carry an extra layer in case I misguessed, or of the weather changes unexpectedly. I can always remove layers if I get too warm. And in emergencies I have my street clothes, in the backpack, although I've never done that.
At 55years old my hands and toes get colder faster than I remember at 30, and that seems to be my limiting factor at 10F. I know I could get chemical or electric hand warmers, but Colorado Springs doesn;t get as cold as Toronto or Chicago, and had fewer truly arctic days.
The three biggest things that have helped me biking in the cold are: 1) Dress Less (As described above).
2) Loose Shoes. For my coldest rides I will wear cotton socks over wool socks and loosen my shoes and toe clips to allow for better circulation. (Similarly, loose gloves or mittens are warmer than tight gloves).
3) Anti-fog for the glasses. I just use soap. A few years ago, after scratching up my glasses I decided to start cleaning them with warm soapy water and air drying them, either with a hand dryer, or blowing on them with an empty pen tube or pinched straw. I just use hand soap, either bar or liquid form. And it's usually so dry in colorado in the winter I can just lightly blow on them. WHat happens is the soap leaves an light invisible film that acts as a surfactant which causes water to bead up and inhibits fogging. As I pushed my cold weather threshhold town to 12F and then 10F I noticed I didn't have the fogging issues as badly as I used to have, even with my belaclava over my mouth and nose. (Oh yeah, a light nylon belaclava just to break the wind, not anything heavy).
The bottom line is, the older I get, the more I realize everybody is different, and there is a wide range of tolerance levels, which change over time as we age. I say push your limits until it's slightly uncomfortable and see if you can adjust to that...and then push a little more another time. At some point you will not enjoy any of what you're doing. That may be a good place to set a limit; although there are some folks that aren't happy unless they're making themselves miserable. And if you're in the 50+ Forum, you are probably wise enough to avoid attempting anything too dangerous, although once again, there are some folks that will tempt disaster until disaster wins.