Noctilucent clouds (NLC) are an atmospheric phenomenon, that is known for only 120 years. In former times this phenomenon could probably not exist due to different atmospheric conditions.
Noctilucent clouds consist of tiny ice particles in the mesosphere, that form at very low temperatures of -120°C (~150 K) and less at an altitude of about 83 km. If the ice particles reach a certain size, you can see it as a noctilucent cloud on clear skys at the proper time. People can only see it at night, if the ice particles still get sunlight at high altitudes but the observer is already in the dark of the night.
This happens about 1½ hours after sunset and before sunrise, respectively. The very low temperatures, that are necessary for the development, only exist during summer in the mesosphere. Further south noctilucent clouds are usually nonexistent. You can observe noctilucent clouds if any only during midsummer (from mid-June to the end of July) and preferably at latitudes of 54° to 58°. Further north the bright nights are responsible, that NLC are not observable.