If you could afford it, how often would you bother to buy new bags, to be safe? I am guessing they degrade a bit in UV light?

The bags in Exequisite are about 20 years old I suspect. When Sonia and myself capsized last year (in no wind, waiting for a start....) the boat floated and the bags held firm. The same cannot be said for my pride...

I replaced the bags in Heaven Sent before this year's champs as they both were going down rather gradually & they looked a bit tatty.  I'm pretty sure they were the originals so 20 years is probably right.   The new ones did the job, fully tested.

I've recently installed some which looked pretty new and found they went down slowly, so I repaired some really small pin holes along the seam with aquaseal which worked really well on those leaks. Then I tested them again and found more leaks, which I repaired and tested again.  This time I found more still, and concluded the bags are totally shot and for some reason they are leaking along the seams - curiously the leaks are all on one side of the seam on both bags.  Maybe an over inflation at some time? too many capsizes?  Probably shouldn't be either as that's what they're built for but with some UV degradation thrown in who knows.
My tuppence worth is if they don't lose pressure they're still OK.  As soon as they develop small or persistent pressure loss, replace them.  I don't know if a 'best before date applies to bags.

It will depend on several things, how often the bags are exposed to light and whether they have bag covers, how the boat is stored (outside or in the dry) and finally abuse of use. I would be weary of bags that do not hold pressure.

I have some in 235 which are well over 30 years old but as other posters have said it depends on UV exposure and how much use has been made of them. In 235 they are well protected by wide rolled in side decks, which, with a dry capsize, are seldom required to perform anyway!

Twenty years? Crikey!

Mine look very faded indeed, but I don't believe they are leaking. I was worried that UV degradation would be enough for them to pop/rip when fully immersed (which is very different to them being surrounded by air). Of course, if I did a buoyancy test they wouldn't pop THEN!