It's likely the Golden Age of Guitar Production. For Martins, Pre-WWII, for Gibson / Fender, from '57 to '65. Things begin to decrease in price accordingly with condition, year of manufacture, and originality. (i.e. you gotta know how to identify something from the used market as having all-original parts, whether any work has been done on the frets or electronics, or refinishing, etc.)
Although I'm not well-versed in the used Golden Age market, the resale value as an investment piece will likely climb as boomers die off and estate sales offer their collectibles back into the used market. The alternative is that those who purchased collectibles several decades ago can expect to see their investments pay off if they were to offer these in the open market, or better yet, at auction houses like Sotheby's where the next generation of collectors will pay well to obtain the unobtainium.
My feeling, though, is not focused so much on guitarists these days as it is on vocalists, and why having a great singing voice can be your ticket to celebrity. That being said, I can't personally sing and play worth beans yet, and am hoping to put more practice time in with my guitar so my skill level can hang with the house band at open-mics. There may be a time where I can learn to sing and play, but realistically, you gotta learn how to do one before you can learn how to do both at the same time.