Funnily enough I've been pondering posting a question about whether anyone had an instrument that got worse with age, things like loss of volume and thinning of sound particularly. Is this more liely to be string related?
Out of dozens of solid-wood instruments (guitars and ukes) that I have owned exactly two
very clearly opened up radically and unquestionably during the first several weeks to 6 months I had them. One was a Seagul S6 cedar-topped guitar and the other is my Mainland mahogany soprano. Both went from "good enough that I bought it after playing it" to "holy cow I can't believe this thing" in a relatively short period of time.
Note that in spite of personal experience with the two instruments cited above I am very skeptical of most "opening up" claims and I would
never buy an instrument on the speculation that it might open up and get better. If it's not good enough to buy right now my money will stay in my pocket, thank you very much!
I've had other instruments that
may have opened up more slowly, but I wouldn't swear to it and wouldn't swear that it wasn't just the factor of my becoming more familiar with that particular instrument and how to get the best from it. I know that I have played brand-spanking-new Taylor guitars that sound every bit as good as my well-used 13-year old Taylor - as near as I can tell it hasn't changed a bit since the day I bought it new.
I have had exactly one instrument that I
suspect has gotten a
little worse with time in spite of new strings, though if so it's subtle enough that I couldn't swear to it. Also, that instrument started out amazingly good and now seems only very good, so it could be simply a matter of comparing it to a different stable of instruments than were around when I first got it. It would be difficult to say with any certainty either way.
Finally, regarding vintage instruments and the supposed fact that they sound so good because the wood has opened up (or the alternate claim, that the wood was better than that currently available). In some cases there may be some truth to those statements, and also a certain amount of truth to the idea that they were probably more carefully crafted by skilled workers than the "average" modern instrument. However, I think it's important to consider that we're looking at a fairly small sample of surviving instruments and it's quite likely that the reason they survived was that they were the "pick of the litter" to begin with. Their brothers and sisters that were only average have long since ended up in landfills.
John