If Letterman really was talking about the 18yo daughter, then he is even more stupid and irresponsible, and has a staff full of idiots. The guy makes millions of dollars a year, has a national TV show, and a huge staff of writers. Am I to believe that not one of them reads a newspaper, and were completely ignorant of who was traveling with her when they decided to write that joke?
Hardly. He was implying that the 14yo is following in his slutty mother and sister's footprints.
He should be pilloried.