I have a KTC-2 that's about a year old, and I've had an older (about 10-12 years) KTS-6. I've also owned an older FS-5 and currently have a KS-5 (they are the same, only the brand is different). Additionally, I have a friend with an older Kiwaya mahogany concert.
I don't think the construction has changed on the KT models, but I haven't looked at the most-recent ones. The laminate models have had cosmetic changes, and are CITES compliant, so a walnut fretboard. Inside they're the same.
My friend's concert is a little different than mine (and more expensive). It has a slothead and a smaller, less deep body. Neither of us likes it as much as the KTC, but that could just be the difference between two instruments.
The KTS-6 was the prettiest, best-sounding mahogany soprano I've seen, owned, or played. But whether that's due to age, I'll never know. My KTC-2 is the best-sounding concert I've owned or played, but i have less experience with concerts. This is my second and I haven't shopped for others since I got it. I also think it has an understated beauty, and the finish is so perfect you just want to pet it.
The older FS-5 sounded somewhat better--or at least different--than the KS-5. Again, no way to know if that's due to age.
So I'd say if the price is right and you like it, get it, you won't regret it. If wood really improves with age, you're ahead in that respect. If not, you're still getting a beautiful instrument, but at a reduced posted. Win-win.