It's really a matter of comfort. In a tee-shirt and jeans, with no windshield, you'll get really cold and it won't be fun. But with a warm jacket, maybe with a liner or a sweater under it, and warm gloves, you might be quite comfortable. I often ride in the 50s.
I think the hard limit is freezing. If it's below freezing there could be 'black ice' on the road. I don't know how people handle that on a motorcycle, but I've known a number of people who have ridden on ice and snow.
A friend and I had a plan to go over the Sierras (here in California) a couple years ago, to a maximum height of about 9500 feet. He couldn't go until the 1st week in October, and then we rode all the way across the Central Valley (about 100 mi.) only to find that the pass had been closed by snow. We decided to have lunch and ride home. But by the time we were finished with lunch, the pass was open again. So we took off.
It turned out the pass had just been plowed. There was a foot of snow on the ground all around us, and I was scared to death we'd hit patches of ice on the road. But we didn't. The next morning it had snowed again and our bikes were covered with snow! And we came back over the same pass.
The thing is, despite the snow, despite the cold, I was never uncomfortable. I wore my all-temperature jacket (mesh shell, thinsulate liner) and my heavy gloves and a balaclava. I actually enjoyed the cold temperature!
Also having a windshield or fairing makes a HUGE difference when it's really cold. It's that blast of air in your face that makes you cold. 8^P