Speaking somewhat from experience, I've purchased two guitars that were new, old-stock (NOS), and I haven't noticed any discernible difference between their sound quality and a newer guitar.
Now, one of the guitars of which I speak is a Liberty Tree Guitar, so it's a bit harder to make a one-to-one comparison there, since the back/side wood is unique. But the guitar sounds pretty good to me, regardless. That guitar was a 2002 issue, and I purchased it late last year -- approximately 11 years old. My understanding (through my correspondence with the authorized Taylor dealer) was that the dealer had kept for display purposes in their store, and only decided to sell it around the time I purchased it. I don't know how much it was played while it was displayed, but it doesn't look like it was handled very much 'cause it looks like a new guitar.
The other was the T5-S with the Aztec Gold top, which was a 2008 model that I purchased earlier in 2013 (5 years old, almost to the day), also from an authorized Taylor dealer. I simply bought it as an interesting companion piece to a '98 Corvette that I own with the same color of paint (well, at least the names are the same; the actual color shades are a bit different; see attachment). Anyway, that guitar essentially sounds like any other T5-S I've played.
Based on what I've read, the 'opening-up' and 'tone maturing' process starts right away and progresses with age; playing the guitar essentially just speeds that process along by 'limbering' up the wood, in particular, the top wood -- making it more responsive. It's probably analogous to exercising/flexing the human body: little by little, it gets better.