Take a look at this:
http://youtube.com/watch?v=OtO48w5w_U8
This is Kumrom Vaitayanon (Master K), working with Kaensak Sor.Pleonchit this past weekend in Sewell, NJ. Master K was a professional fighter in the 50's/60's, with over 70 professional fights. He will be turning 70 years old later on this year.... Doesn't look like MuayThai has had any adverse effects on him at all!
You have to take some things into consideration. Master K knew when to call it quits (ring fighting), but he maintained a daily training regimen. He takes care of himself. He doesn't smoke, he doesn't drink. He stays active....
It's not just a sport, it's a lifestyle!