Vintage” is a term that has acquired a new meaning apart from its original usage. The term is a combination of Vint (of the vine) and Age (time of creation). This term is used in the wine industry to indicate a wine’s harvest date. The use of “vintage” has been modified by collectors to mean old, such as a Vintage Car, or Vintage Clothing. This extension of the meaning is used in guitar terminology to mean “an original, older guitar.”
Most collectors value guitars from the mid 1920s to 1970. Guitars prior to the mid 1920s are too primitive in design for most collectors. Guitars after 1970, even though they are over 30 years old, have little collectible appeal. All the U.S. guitar manufacturers were in dire straits during the 1970s. They were either bought out by larger conglomerates looking to make guitars as quickly as possible, and/or their quality and choice of materials had become sub-standard.
And yes, they are seen as stable investments!