I won't play if it's below 40, I know a couple other snowbound THPers mentioned this earlier, but I've tried it and it's really just too much and not enjoyable.
25 degrees is about my limit. When the ground is frozen, its time to put the clubs away. Hitting a shot fat when the ground is frozen is a good way to jack your wrists up.
I'll play out here with anything above the teens and little to no wind. Greens will be solid, but can practice distances and b&r's as well. Multiple thin layers that aren't tight and can still swing in them, I'm out there. With it being a dry temperature out here, it is not really all that bad at times. This winter has been more severe than most (not like the east coast monsters), so haven't been out like last winter. Is never a good game with the temps that low (lack of body limberness), but still fun to get out as long as I remember just to take it easy.
As an golf addict in grad school at Penn, a group of us would rent a car every Saturday and Sunday and drive to the only course in the Philly area that was open all year -- Swedesboro Golf Course. We had to cross the Delaware River on a bridge that had a flashing temperature sign on it. The rule was we'd turn around only if the temp was below 20. Many's the day we'd approach the bridge chanting "Come on Twenty!"
If we got a twenty, we'd stop at the first Jersey liquor store and stock up on hand-warmers and peppermint schnapps. Had to play low compression colored ladies balls to find them. Snow drifts were played like sand traps. Usually they were so frozen, balls wouldn't bury. I've seen balls stick to a slush covered iron face like a tongue to a flag pole, 300 yard three irons bounce down fairways you could skate on, and my first backwards wedge hit high into a gale that started 40 yards away and ended up 10 yards behind the first shot. And there are advantages. Your driving distance will go up. Water hazards lose their powerful to terrify when you can walk across them.
Anything above 20*F, as long as the its mostly grass, or at least looks like it, and not snow covered. Patchy is just fine, Sun helps but not necessary, a day walking 18 is better than a day not at any temp
If the temperature starts with a "3" I am checking the wind situation and playing if it's not too bad. If it starts with a "4" I am already on my way the course.
If I waited til 50 degrees, I wouldn't get to start playing until May.
My first round this year so far was yesterday, and it was 35,6 degrees (2 degrees Celsius). It wasn't bad, I didn't have mittens so at times it was a bit chilly, but still awesome! On Friday I'll play at a tournament, predicted temp is 36,5.