Think of a very old person on stage or screen, and what do you imagine? Perhaps a character doddering slowly with a cane, wizened and wise. Or maybe downright cantankerous. But how about an older musician – do you picture them wailing on an electric guitar? A new arts festival has been launched in London, and it’s upending the image we have of older artists. The BBC’s Paul Moss meets some of the performers at the “Age Against the Machine” festival.