I train at ~5:30am most days in my unheated garage - it’s usually okay down to about 30-35F, just means I warm up without fans on for a bit longer (though I always turn them on before the first hard interval). Anything below that, the fans tend to freeze my fingers and toes, and I move my rollers inside. That beings said, I’m pretty over training in the cold at this point…so lately I’m moving indoors at more like 40F…I’m ready for warm weather.
Take all the above with a grain of salt - I live in North Carolina, so it’s not like we have bad winters to begin with.