Yes, this thread has certainly generated a lot of interest and rightly so. I'll add my 2 cents here...I bought a 1995 612c new at that time. After about three years I noticed a clearly discernible change in its tone. The bass and low midrange became fuller, it had more punch, and it just sounded better to me. That guitar is now 17 years old and sounds very nice. My son has it in Arizona (yes, he knows to keep it well humidified).
My 2008 RT2 has gradually matured, tone-wise, over the last three years as well. I have already noticed a change (for the better) in the tone of my new RT2.
Sad as it is to say, I don't think my playing has improved over the years, so that's not likely a reason for the changes. I have no idea why this occurs, and other than purely for academic interest, I really don't care. It is great though that acoustic guitars, at least well-built ones, go through this process. I'm hanging on to my guitars. The best is yet to come.
In a recent W&S, Bob Taylor was responding to a question about this very topic - he was talking about people who buy and sell guitars without keeping them long enough for this phenomenon to occur. He says it takes at least 8 to 10 years for this to really develop. I tend to use this fact as an excuse to buy new guitars now before I get so old I won't be able to play (or hear) them very well. I'd insert a smiley emoticon here if I could figure out how. Any one have any helpful suggestions.....
Bill