It depends. Your version of  too cold might be different from someone else’s: it all comes down to experience level, equipment, and tolerance to the cold. If you’re completely new to winter camping, you might want to ease into it by not going in the dead of winter, maybe opting for the shoulder season first instead. A seasoned mountaineer, on the other hand, may be comfortable and equipped to dive into temperatures as low as -40 degrees Celsius. Ultimately, there is no concrete “too cold” temperature, but make sure to always check the forecast before you go to know what you’re getting into.