Listed buildings have nothing to do with the date they were built:
How are buildings chosen?
Buildings can be listed because of age, rarity, architectural merit, and method of construction. Occasionally English Heritage selects a building because the building has played a part in the life of a famous person, or as the scene for an important event. An interesting group of buildings - such as a model village or a square - may also be listed.
The older a building is, the more likely it is to be listed. All buildings built before 1700 which survive in anything like their original condition are listed, as are most built between 1700 and 1840. After that date, the criteria become tighter with time, so that post-1945 buildings have to be exceptionally important to be listed.
Also, just because something is listed,it isn't automatically protected. Try Google, there is lots of information out there.