For the last few years there's been a fair amount of construction near or along my commute. It serves as a reminder that as cold as I might feel on any given morning, it's only for about 45 minutes or so. There a lots of people outside all the time. My hats off to them and to you. I'm not sure I'd want to ride a bike in the clothes that I'd have to wear to stay warm all day.
I don't have a temperature cutoff though. I've ridden into the -20s. Like Spid if the snow gets too deep I won't bike. When it's easier to walk than it is to ride, it doesn't make much sense to me.