What is the life span of a purple tang?

it is said in some websites that it is 5 and in others that it is 20 or more but they are also talking about the blue tang with a tank life not out on th reef. So it is really unknown.

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I've had my Blue Tang(Hippo Tang) for 11 years in a 55 gallon aquarium. I bought it as a juvenile (about the size of a silver dollar). Now it's about 7 inches long and doing great.