Humans are essentially tropical animals and are not equipped to deal with even mild cold. That we can live in cold climates is a result of behavioural adaptations such as wearing appropriate clothing and building shelters.
Successfully surviving cold requires two simultaneous events. Firstly, generating sufficient body heat by burning food and secondly, preventing the loss of that heat by suitable clothing and shelter.
The lowest temperature that can be endured is a combination of the duration and the extent of the exposure, so it is not easily determined. A naked person will start to feel cold if the surrounding temperature drops below around 25°C (77°F). Physiological responses such as shivering and diverting blood away from the extremities and surface of the skin will then kick in.
These physiological responses mean that a reasonably well nourished adult can maintain their core temperature in still air just above freezing point wearing only light clothing, though they will feel cold and shivery.
Normal human body core temperature is 37°C, 38°C is a fever making you feel very uncomfortable and 40°C is life-threatening requiring immediate medical intervention. Below normal temperature, at 36°C you will start to shiver and feel very cold, by 35°C the cold feeling is very intense, the skin may feel numb and have a blue/grey tinge as a result of diverted blood flow. The range of body core temperatures we can function properly within is very narrow. Perhaps the most surprising thing is that most people go their entire lives of many decades without once leaving the normal zone by more than a couple of degrees either way.
