Actually, there are chemical changes in wood over time. A 50 year old piece of lumber will be crystallized a bit, relative to wood that was cut more recently. I'm not sure what it does to the resonance or anything else, but I do know it will be more stable. It will also be lighter, since newer lumber will not have had time to fully dry like older lumber has. If you went back to 1961 or whenever and picked up a P-Bass and then took your time machine to now and picked up the same P-Bass, chances are it would be lighter now than it was then. Not by much, but it would be.
Wood now is typically cut younger and from less dense stands. That means the grain will be a bit coarser and the wood a bit less dense. That goes counter to the idea that older wood will be lighter, so pick your poison.
Anyway, wood does indeed change over time. Whether it is better or worse is your decision, and wood is a little different now than it was 50 years ago. It will be different again in another 50 years, so if you want, you can put a new P-Bass under your bed and then pull it out in 2065 to see what it is like. That will settle the arguments. Time is a small price to pay for winning an argument, right?