I rode about 50 miles last week when it was 16 degrees F. and I wasn't cold and recently rode 100+ miles at 28 degrees(I have ridden in -7 degree weather). The key to staying warm is layers (lots of them). I wear two or three pairs of socks, waterproof very cold weather boots, long johns, pajama bottoms, jeans, thick flannel shirt, Wall's Work Gear brown insulated overalls (like farmers wear), three coats (a thin jacket, a down coat covered by an extra large leather jacket, a bib to protect my neck, a full face helmet, ski goggles to protect my eyes, thick gloves/mittens and I turn my heated grips on high. Also, I have found that the wind protection provided by my RT with the windshield all the way up is amazing --- it feels like it is about 10 degrees warmer than the actual temperature!
This may sound like a lot of clothing but it is very comfortable. However, back in 1974 I rode 850 miles on a 350 Honda in 40-50 degree temps and about froze to death. I was young, dumb and woefully under dressed.
In June I'll be 66 and would hate to ride only on days above 60 degrees or some other arbitrary temp. Riding is the cold greatly extends my riding season. The only time I won't ride is on ice and snow.
Riding in the cold can be fun.