I'm not sure that 'average lifespan' estimates are all that useful unless a person gets very clear on what examples they're taking to be included in the average. I doubt any are based on actual data, and so they're probably not a mathematic mean of all captive specimens.
They likely don't factor in fish that die in the collection pipeline before they get to the hobbyist's tank, for example, which would drive the mean down significantly. Think about fish like Moorish Idols or Orange Spot Filefish, which have a potential lifespan of at least a handful of years but a mean of probably much less than one.
Whatever estimate a specific source gives surely is based a lot on what that source takes to be a relevant data point.
All that said, I had a Yellow tang and a Scopas, both WC, that were with me for 16 years. Not even the same genus as you're asking about, but I don't know of any reason the expected lifespans would be all that different.