Guitars are often said to sound better with age. How much of it is due to worse-sounding guitars being discarded before they reach vintage age?
It is possible that some guitars just never live long enough to become "vintage", because they never sounded good in the first place. And a bad sounding guitar is not going to improve much with age. In the case of a laminate top cheap mass produced guitar, no amount of aging is ever going to make it sound like a new solid wood guitar.
Acoustic guitars have proven to the ears of many players - to sound better as they age. The theory that best explains this is - that as the wood in the body ages, it becomes lighter and more responsive and more resonant.
In an acoustic guitar, the sound comes from the vibration of the top - also known as the sound board. The vibration of the strings alone produce very little sound. The vibration of the strings is transferred to the top through the bridge and saddle. The vibration of the soundboard (top) is what produces the audible sound we hear from an acoustic guitar.
To prove this, take a tuning fork and strike it to set it into vibration. It makes very little sound on it's own. But if you touch the end of the vibrating tuning fork to the top of an acoustic guitar, the note created by the tuning fork rings loud and clear.
Some folks believe that the more an acoustic guitar is played, the better it sounds because the vibration of the top loosens up the wood and leads to greater responsiveness. I have heard that some guitar owners will set their guitars in front of their speakers and turn up the volume on their home stereo believing that the vibrations will help loosen up the top and make it sound better. But I think this vibration theory is more like speculation and difficult to prove.
Yamaha is convinced that aged wood has superior tone producing qualities so they developed an accelerated aging process knows as Acoustic Resonance Enhancement Technology (A.R.E.). They started using it on violins and have recently began using this process on some of their acoustic guitars. You can read all about it and watch a video here on the Yamaha Website Acoustic Resonance Technology by Yamaha
Another reason "vintage" guitar's may sound good is that the wood available decades ago may have been a better quality wood than what is available today. It's also possible, that older guitars from reputable makers, were more meticulously crafted with more of the work done by hand and more time devoted to finding the best sounding wood for the soundboard and more time tweaking the building process to get the best sounding guitar possible.
Mass produced guitars - even by well respected companies such as Martin, Gibson and Taylor, don't get the same amount of individual attention as they did in the "old days".
Most of the foregoing relates to acoustic guitars sounding better with age. Electric guitars which rely more on their electronics to produce sound, are less likely to show any dramatic improvement in tone through the aging process.
The factors discussed above all offer plausible reasons why 'vintage" guitars seem to sound better than newer guitars. And certainly - a really good sounding new guitar is more likely to live to become vintage due to receiving better care.
But I believe that tone woods used to make acoustic guitars, do improve their sound producing quality as they age. Different tone woods have been shown to improve at different rates - characteristic of the particular type wood (Spruce, Cedar, Maple, Rosewood, Koa etc. will all age at different rates.
And I am convinced that a given well made guitar that sounds good when new, will (if well cared for) sound better as it ages.
In some circumstances the vintage instruments sound better due to changes in the manufacturer's specs. For instance, the Fender P-Bass is a very highly desired instrument from certain specific years (can't remember which years off hand). Fender changed their specs and many have argued not for the better. In this particular case, I think there are two factors. The obvious one, the change in sound due to manufacturing specs. The other consideration is that of the history of the instrument. Many great artists used these classic P-Basses on their recordings that have become standards in the modern repertoire. One could argue that due to their consistent use, the sound of that particular series is considered the P-Bass sound, so if you want to emulate the sound of your favorite Fusion bass players, then you are going to want a P-Bass from those years.
In the case of acoustics, age improves sound as the wood becomes drier. As for as electric guitars, age has little to do with it. A well made new instrument can sound just as good, provided it is made with quality tone woods. There could be some argument that older electric guitars were made from old growth wood that is simply no longer available.
I think something to consider, is the the wood might have more slight imperfections over time, due to constant environmental changes (temperature, moisture, etc...) and that these imperfections create a fuller more resonant tone. I agree with most that a guitar that never sounded very good... won't sound better over time, but a great solid wood guitar, will sound better over time .