How's your facial hair? Seriously, down to about 5*F, I don't cover my face, but I do start to use a neck gaiter around that point, in part because I add a beard for the winter. I always have eye protection: glasses (sunglasses or safety glasses) down to about 20-25*F, then ski goggles below that. I also use a fleece ear warmer (the band style) starting at 40*F. I have a helmet that easily adjusts (it has a knob in the back that loosens the diameter), which makes a big difference. If it gets truly cold, I swap the ear warmer for a skullcap or my
Outdoor Research Peruvian hat, which is windproof. In other words, multiple options that depend on the severity of the weather. I don't generally like balaclavas or the like because they insulate two regions of my head (neck/face and ears/top of head); I prefer to control the layers on each part independently, though some would argue that the gaps between the pieces is an issue.