Road conditions are probably the limiting factor. I don't like to ride on ice or in slush.

My coldest ride last year was after New Years on mountain bikes up at Fair Hill preserve in Elkton, MD. I think that the wind chill was -17 degrees and the air temperature was just below freezing. Actually, it was a lot of fun, except where we had to cross smooth frozen puddles (scary) for a few feet at a time.

Anything above freezing isn't a problem if you know how to layer and have proper gloves, I think. PG