The Guardian newspaper is reporting that Jack Nicholson, 76, is retiring due to memory loss. He's not able to memorize lines.
There are ways around it. Cue cards were originally used to help older actors. Marlon Brando used an ear piece, like the one George Bush, Jr, used to cheat in the debates. Someone would read his lines into his ear and he would repeat them. It takes a little practice to sound natural, but if George Bush, Jr, can do it, Jack Nicholson ought to be able to. Brando used cue cards early on. He used them in Last Tango in Paris in the early '70s.
Brando was such an oddball. There were reports that he wouldn't wear pants on the set. He was terribly overweight and didn't want any full body shots.