HOw to fire an old employee?

skp the bullcrap, fake job responsibilites and get straight to the point of the job. I'm a supervisor for a big name water companys. We delivery water to residiential and commerical property. There is an employee who has been with us for twenty years. However, he is now old and beat up. He can't keep up with the young, fresh, and non beat up guys. PLus, the new guys get paid almost half the hourly rate as the old guy. I know by law, I can't fire him because he is old and beat up. Is there a way to get him to quit or set him up to fail so I can legitamelty fire him. I don't care about his life outside the workplace.. I only care about profits and my bonus. Any help is appreciated!