Growing up in Canada I used to ride all winter. Studded tires, ski mask, and heavy duty wind-proof mittens. Oh, and thick tights. I could do about -5F very reasonably. Never got much colder than that in southern Ontario.

Oddly, a few years in California and now I can't stand the cold but love the heat. I like biking in 80F weather and am ok up to about 100F if I slow down. Hotter than that starts to seem a bit pointless.