Where I live it gets into the teens (F) and, very occasionally, the single digits. I don't ride when it's in the single digits, but for the colder temps a balaclava is essential. I take off my glasses when it's very cold and use ski goggles (keep the glasses in a case in my pannier). This might not be possible if your vision requires glasses at all times. The ski goggles prevent my eyeballs from freezing, especially when the wind's blowing.