I have 3 silver dollars (Metynnis maculatus) that came to me in December 2017, when they were a documented 18-20 years of age. The documentation is that there are records of them being kept at the place for 18 years, but they were probably already ~2 years old when they arrived. 2017 to today makes them 24-26 years of age. They look mature for sure, but not declining or particularly old in any way.
I also have a stripped headstander who might be 8 years old, and I recently lost a flagtail Prochilodus (not to senility) of probably the same age.