I’ve seen this topic be argued about endlessly.
Ultimately the answer is up to your own level of risk/reward. New belts don’t cost THAT much. The consequences of a failure will likely be catastrophic and very expensive indeed.
Ducati used to recommend 2 years, then 3 and now with newer bikes, 5 years. I don’t really understand why anyone would choose to just ignore this?
The belts are under very high tension, like a bowstring. So the reason age matters as well as mileage is due to that. A bike sitting still for a very long time, may have nice shiny clean belts but that high tension has been exerted on the same part of the belt for maybe years. Just because you can’t see the wear, doesn’t mean the belt hasn’t slightly stretched and doesn’t mean now when used, it won’t deteriorate very quickly. A guitar string left unused for years, will stretch and go out of tune much quicker than a nice fresh one.
You don’t want to be out on a long ride to find out your belt is now wearing significantly as the rubber is ageing or it has stretched at a certain point.
I don’t believe a visual inspection cuts the mustard here and answers the question. Could you simply look at a set of 10 year old tyres and tell me how old they are…? Sure, no cracking or splits but I still wouldn’t want to use them.
It’s your bike, your choice but personally… I would be MORE concerned about a bike that has done so few miles in its life, not less. Bikes don’t like to sit still.
I would be replacing as many consumable parts as possible. Belts, tyres, fluids, bearings etc…