In general, webbing is webbing, although it can be made to different strengths/thicknesses. For example--we're not using webbing that is of the same thickness as we would find on a child restraint, in our vehicles. Webbing abrasion, colourfastness, and various use/exposure scenarios are tested under FMVSS in accelerated testing. So just like carseats, it's all required to meet a minimum standard. The question is this: How do we translate this testing into a reasonable lifespan for seatbelts? {Also noting that seatbelts are a
system, and we're not just looking at webbing exclusively--your seatbelts include webbing, d-rings, buckles, latchplates, retractors, etc...}
Having said that--I do have a "classic" vehicle, so seatbelt expiration is something I have looked into. Seatbelt expiration recommendations run the span of 2 years (for race car drivers) to 25 years (classic cars), at least in the classic car/hotrod circles here in Vancouver. Most of the hotrod/kit car folks have add-on belts (since belts weren't present in the original vehicles) if any at all, however. So this isn't exactly a common concern with people who have older vehicles, since they tend not to be daily drivers and most of the drivers with classic cars are older and didn't even wear belts growing up.
My 1989 Landcruiser is 23 years old, and I do believe that the seatbelts should be replaced now or soon. First, I inspected the belts for damage--and although there's a little bit of fraying and nothing really obvious with the belts, the webbing actually feels differently than newer car belts. The webbing feels a bit like it's stiff and "melted" to itself. It's not soft or pliable. This is probably contributing to the fact that the belts are not being picked up and spooled in by the retractor very easily. My retractors also seem a little slow on emergency locking. It's very difficult to trigger the emergency locking feature. Furthermore, the buckle button on the driver's side is missing so I have to dig down for my buckle these days..right there, this would warrant replacement because the belt is technically missing parts. All of these issues combined mean it's logical to conclude that, at only 138k miles but 23 years old, at 23 years most vehicles will have exceeded a reasonable lifespan for the seatbelts.
I think a conservative number for belts would be 15 years, assuming that they're being inspected and there's no obvious damage to the belt, buckle, or retractor(s). At that point, I would encourage someone driving an older vehicle to inspect their belts every few months for problems. The 20 year mark in a vehicle that has been well cared for is probably approaching the reasonable lifespan for a belt.
My Odyssey is just over a year old, and the driver's belt is fraying.. so you just never know. If you haven't owned the car since new, you also don't know what might've been done to those belts over all of those years. That may factor into your decision with regards to seatbelt replacement.
Most import manufacturers will replace belts under lifetime warranty. I know for a fact that Honda is absolutely wonderful about doing this. Apparently the domestics haven't been as good about it--since I know two people lately who've tried to have seatbelts replaced as a result of broken cinching mechanisms only to be told that the belts aren't covered beyond the original warranty period. Both were Chrysler products. Obviously this isn't in the best interest of the vehicle manufacturer, as far as liability goes.
My official answer on the lifespan of seatbelts:
You should replace your seatbelts if you spot any damage, they have missing parts, or the parts are not operating as intended by the manufacturer. Older belts should be replaced as per manufacturers' instructions. It is important to inspect your belts for damage and malfunctioning parts on a yearly basis, increasing to every few months once the belts reach the 15 year mark. The reasonable lifespan of belts will depend on use, abrasion, exposure to chemicals, and a variety of other variables that cannot be accounted for in a single lifespan number that is applied to all vehicles, belts, or situations.
But..just my opinion.
-Nicole.