Age will eventually do for a belt, but mileage is usually the biggest factor.
Bearing in mind VW cant make up their minds I intend, as a low mileage user like you (lower than you in fact) to go at 7 years, split the difference between their old and current advice, and will use OE kit.
Strangely Gates seem to have a good rep for their parts in Transporter circles, yet in the Volvo community where I also reside theyre avoided like the plague. Who do I believe? The only sure answer to that one is to use the same parts that the manufacturer did when the built the van, and to have it installed by a business that will a) provide a decent warranty, and, b) is liable to be around long enough to honour it.
Interestingly, 5 cylinder Volvos have a 10 year belt interval and if the job is done by the dealer they warranty it for the full 10 years. VW won't do that. Anyway, thats my Sunday moan.