the cold weather, and is there a lower limit for temperature?
I think you'd get quite a range of answers from different people. Everyone's tolerance for cold is different. I start to feel like it's not much fun below 25 degrees but others would have different numbers. I used to think 35 was too cold.
Lately I worry more about conditions for the temp. Last year on Thanksgiving day a friend of mine broke his hip on an ice patch at the intersection of Middle and Harts Run roads. That intersection is often wet when it's been raining or snowing. Sometimes it has been heavily salted but not that morning. It wasn't much of a fall but he was strictly no-load on the joint at all for six weeks, followed by weeks of rehab. The good news is that it healed up just fine and is back to kicking my A$% up hills.
But it made me start to assess how likely I am to run into ice etc. in addition to the temps. 25 on dry roads? No problem. 29 the day after it rained/sleeted/snowed a lot? Probably not.
Others (see the thread on changing weather) use studded bike tires to beat the ice. It's all what you are comfortable with. But don't let the cold alone beat you, with some wind blocking and layers, you can probably ride in much colder temps than you would think.