One of my favorites is Jimmy Buffet's "The Captain and the Kid." Also, Simon and Garfunkel's "Bookends."

I suppose the protagonists in "Hobo's Lullaby" and "Mr. Bojangles" (both are in the database) are not all that old chronologically, but I'm inclined to include them anyway, since they seem to have lived too long for the demands of the open road.