Can it be too cold for a jet to fly? Not really. While it cannot be too cold for a jet to depart or fly, frigid temperature extremes can cause problems or delays prior to flight. As with most machines, there are temperature limitations for operation. Oil and hydraulic fluids must be within certain temperature ranges prior to departure and frozen brake lines can plague aircraft as well. Ground support crews are trained in warming aircraft and certain components, such as engines (turboprops), prior to flight.
Engine starts in very cold temperatures can be problematic as well, though less common with computerized engine management systems. Additionally, any contamination from ice or snow of flight control surfaces or wings must be removed prior to flight. After de-icing, time tables must be adhered to for departure depending on the type of fluid used, or de-icing must be repeated.
Once airborne, aircraft function very well in extremely cold temperatures. At altitude, temperatures of -100C can be routinely observed and airplanes handle this without a problem. So, while it can’t be too cold for a jet to fly, starting and warming up the aircraft can cause some additional nuisances or delays.