Took the boys to the vet. They're getting old and needed a check up. We were told they are still quite healthy, so that's good.
They're at the age were impaction usually starts (anus muscles not being strong enough anymore to push poo out, so you have to scoop it out manually), so we eventually have to look forward to that.
They also like to laze around a lot more as of late, rather than moving, so they're getting fatter as a result too.
I'd say they'll live to be at least 6 years old, but I think we need to make some adjustments so they're more active. I've seen guinea pigs reach 10 years old, but it is the exception rather than the norm.