For "mature" audiences only

I was shocked this morning to see that the MTCRB had decided to award an R-13 (Restricted to ages 13 and up) rating for the movie "Sex and the City" instead of a more suitable R-18 (Restricted to ages 18 and up). (In the US, the movie was rated R, under 17 requires accompanying parent or adult guardian.) Although it is possible that the more objectionable scenes had been edited out here, do the members of the board really think that it is appropriate to expose our thirteen year olds--and without an adult to accompany them--to the overall theme presented by the movie?