The Effects of Aging
It is often said that, as a guitar ages, it's sound improves. This is largely due to it's finish breaking down over time, becoming brittle and thin (and, not too shiny in the long run). New polymers hold up much better over the years than old-time lacquers and will retain their gloss as long as the guitar is well taken care of. Some controversy exists as to whether these finishes are as good in terms of aging and its effects on sound. Regardless, a guitar that sounds good in the store will good at home despite its age.
Wood ages as well; however, old wood doesn't necessarily sound better than new wood. Guitars, after years of vibrating tend to become more flexible and, as a result, more resonant. Constructing a guitar of old wood still yields a new guitar. It is only after years of being played that the guitar evolve in this way.
All that said about aging, be careful not to assume that old guitars are better than new guitars. Manufacturing processes are better now than are better now than ever. Guitars are being made of the highest quality in the history of the instrument. Today's new guitars often surpass the sound quality of even the most highly prized vintage instruments.
Imagine what these guitars will sound like after twenty years of playing!