The Simpsons cast vs aging.

I came home, watched a Simpson's episode that was sufficiently bland that I sat down and reimplemented the thing I did for the Supreme Court for a generic data file.  Feeding the main voice actor cast for the Simpson's into it (and quantizing on years because working out partial years would have taken more time than I wanted to spend.

Individually.

Summed.

Based on this, The Simpson's is unlikely to lose any of the main cast until 2020 at the earliest.  Assuming they keep going after that, they can continue for about another twenty years before they lose half of the cast.  However, Harry Shearer is that first line, and he voices about a third of all characters on the show.  That would probably be hard to recover from.