Most Bluegrass musicians try to find that perfect vintage instrument with a really warm, sweet and snappy sound. There is a tradition to uphold the music as well as what makes that sound. Players are using custom instruments more because the makers work hard at creating really good reproductions of the finest vintage instruments. I believe a new guitar or mandolin can sound great within a year or less if it is made right and played in properly. Then it just gets better and better with time and use.
Modern builders have an advantage over the pioneers.
We have the sounds and looks of accepted vintage instruments to guide us specific standards. So I try, when possible, to use old wood, for tops especially. You can get a bit of a jump on the aging effect and a more mature sound. After many years the cell content of wood crystalizes, stabilizes .this is the seasoning that is so important. Less seasoned wood can be used once it is dried for a reasonable amount of time, and the rest of the seasoning can be done while it is working on a finished instrument.