While this is helpful, it can lead to a crowded rare book room. It is more appropriate to use dates which are meaningful to your subject fields. Some libraries consider any pre-1960 imprint rare, depending on the publisher and subject matter. Some libraries house damaged material that cannot be repaired immediately in the rare book room. Usually, this is in a separate area for items awaiting repair, but sometimes a book is formally transferred to the rare stacks.
Some libraries have preservation collections of books in pristine condition.
These are usually special collections of specific importance to an institution. These books would be considered rare. Often a book is considered rare because of the history surrounding its publication, author or previous owner. If the publishing house was small, private or not generally associated with this type of work, the book could be considered rare. A former owner of a title may have played an important role in your library's subject field.