You can exercise outside at any temperature but I would avoid exercising outside if the temperature is below freezing outside.
But if you do exercise outside at below freezing temperatures just make sure you dress appropriately and don't stay outside in the cold for too long.
Be sure to take frequent breaks and go inside somewhere warm before going outside again to exercise.
I've tried exercising outside in below freezing temperatures and I just couldn't stand the cold temperatures so I only exercise outside when the temperature is above freezing.
But it's safe to exercise in any temperature as long as you warm up often and dress appropriately.
But you could get frostbite or hypothermia if not dressed appropriately.