An elderly US couple charged with spying for Cuba for almost 30 years pleaded guilty in the conspiracy Friday, with the husband agreeing to serve a life sentence, the US Justice Department said.Former State Department official Walter Myers, 72, pleaded guilty to one count of conspiracy to commit espionage and two counts of wire fraud, while his wife Gwendolyn Myers, 71, pleaded guilty to conspiracy to gather and transmit national defense information, according to the department.