My catfish

[SheDevil]

Hello
I was just wondering the average life span of an upside down catfish.
I got mine off a friend 15 years ago and he was fully grown back then and i've had him since.
He is in excellent health (touch wood)
I'm amazed that a fish can live this long in captivity and curious as to how long i can expect to have him.
Any info would be appreciated.
thank you
jane