you need to define what you mean by old wood. a 100 year old tree thats cut down yesterday would be "old growth", wheras a 18 year old tree cut down 100 years ago is "old wood".
what gatorpayne described is the latter, wood thats been sitting around a long time.
what alot of builders seek out is old growth wood, which may have different density properties than younger trees.
a real 59 les paul would have been "newly cut" older growth wood. NOW, 50 years later that guitar is old, old growth wood, and possibly sounds a little different than it did new.
ultimately, it probaby doesnt matter much, but if you are going to spend alot of money trying to get it "right", you might want to go for some ages older growth wood.