The most common material used for chimney construction up to about the mid 1980's was masonry ( brick, tile, block, etc.) Many of the masonry chimneys built prior to the early 1920's were unlined. This means they did not have a tile liner on the interior of the chimney to protect the masonry from a natural deterioration caused by products of combustion. If your home contains this type of chimney you should seriously consider having the chimney relined or rebuilt depending on its condition.
The major portion of chimney deterioration will occur from the roof line up where it is exposed to the elements. This will apply whether it is lined or not. All masonry chimneys must have a cap or crown that protects the chimney top from the elements and keeps water from laying on top and penetrating into the structure. Water penetration through the chimney top is the most common deteriorating factor. Most chimney caps are not properly constructed and will require repairs or more likely rebuilding after about 20 years. If properly constructed, and they rarely are, can last 50 years or more.
Water penetrating the chimney top particularly in the northern climates of the United States will be subjected to freeze and thaw cycles, which destroys the masonry. This can occur within a few years if the chimney top is poorly constructed.
The flashings around the chimney, if not properly installed, will also allow water to penetrate the structure causing deterioration.
It should be quite clear that water penetrating the chimney structure through the top and/or the flashing is the major element that will reduce the useful life of any chimney and therefore should be prevented.