I have had my serpent star (purple) for 19 years this month and my brittle star for a little over 12 years. The serpent star is particularly resilient. It receovered from a test in the back of its disk after one of the times it consumed a snail too large for it to disgorge the shell (usually the shell comes out a couple of weeks later ground down to a button
-sized disk), it has also lost an arm a couple of times and regrown it. One of the times the arm wandered around the tank for days after being detached.
It is my experience that our tank inhabitants can live much, much longer than literature gives them credit for. I think they can live a lot longer in our tanks than in the wild given the right conditions.
Who would expect something the size of a clownfish to live so long? Mine is 21 years. It was my first fish after cycling my first tank with damsels (the book I read back then recommended this approach).