I was an MOT tester in Britain for a number of years.
I've seen them fail a few times, doesn't matter where they are made or how well/badly they are routed.
Stainless work hardens over time/flexing and eventually a strand will wear through Teflon/nylon liner.
Doesn't really seem age related, more a operating thing, high mileage on bumpy roads seems to make them fail quicker.
they still last around 5~7 yrs on 'average' mileage bikes, just check for loose strands when you think of it (if one is sticking out, god chance one is sticking in)
BTW they are VERY sharp and will tear your hands up