In extremely cold temperatures tents mainly exist to block off the wind and snow. A tent is generally not designed to keep you warm. Tents are not built to offer the kind of insulation that could stop heat from escaping your tent or cold air from entering your tent. The best way to use a tent in cold weather is to allow air to flow from the outside into your tent and back out, as this will allow moisture to escape the tent. Being wet and cold will kill you much faster than just being cold. In this way, there is no such thing as too cold a camping condition so much as if you have prepared enough gear to camp with. You should, however, not camp in temperatures significantly (18 degrees Fahrenheit/10 degrees Celsius) colder than you have previously encountered. You should also camp with people and equipment you have experience camping with. The last thing you need is for something not to go as planned, placing your life in danger, and you haven’t got the equipment or help to safely overcome your problems.