It is common knowledge that timing belts age and fail, also belt can look just fine and snap off.
This shows that when aged belt is stretched it does break a lot earlier than good fresh belt :
http://www.therban.com/imperia/md/co...beltrecipe.pdf
Same goes with any part made of rubber, they fail with age, every rubber part, hoses, bushings etc. 20 years with timing belt is pure luck or that is what I was told when studing to be car mechanic, but it was long a ago, new belts surely can be lot better as any rubber parts are, still it is their curse that eventually they will fail with time.
edit: There is more of this stuff:
http://www.canadiantechnician.com/se...?intDocID=3049
Information about aging is practically everywhere, so I'm surprised that this would be unheard of?