Kevlar and Nitrile...
VW of North America has not taken an official position, but VW - U.K. has a 5 year limit in the literature... Contitech, one of the major timing belt manufacturers strongly recommends 6 years as an absolute maximum, but... they're in the business of selling belts...
YMMV... A bit of common sense should enter into the equation...
Yuri
PS: my thoughts - at least for Diesel-powered cars: if you're not driving enough to use up the mileage limit by the time these recommended age limits come up, you really shouldn't be driving a Diesel...