I do have 2 other black neons that are 5 yrs old and are just fine...
Well, like humans, a certain age isn't exactly a death sentence. Health issues can arise depending on genetic predispositions.
I'm just saying that 5 years is around the max I'd expect out of a black neon. And unless you raised these from eggs, it's hard to know
specific age anyways. Your other older tetras probably wont last much longer, just how their lifecycles work. But they could...lots of variables to consider.
I wouldn't rule out another factor. Sudden bloating symptoms are hard to diagnose because the direct cause is usually something internal and most of us don't have the tools to dissect our fish to find the root of the issue. Like I said, unless your other fish (especially the younger ones) are affected as well and your water checks out then I think it's still safe to assume that age was a factor. Not much you can do about that but make sure it isn't something else you can control (ie. internal parasites, indigestion/blockage).