ESRB president Patricia Vance has made a ruling on how downloadable content must be rated, and it is hard to argue the choice. Essentially, any content that is released for a game must adhere to its original rating, as she explains below:

If a [publisher] submits a game to us and it gets a teen rating and then [the publisher] wants to add downloadable content to that game in the future, which is obviously happening a lot today, they have to keep the content in the downloadable product consistent with the core rating. It can’t go out of bounds.

If [the DLC] does [require altering the original product], then obviously the rating will have to change. But the consumer is still being informed. That downloadable content is still carrying an ESRB rating and the consumer’s ultimately protected by being informed.

I understand this, as it is harder to regulate downloadable content then it is actual games. If I were a parent I would find myself upset that the game my child convinced me was alright because of a Teen rating had received an update that added mature content. The grey area here, because there always is one, is user made content. If this would also count as downloadable content then game markers are in trouble, as modding their game would instantly award it an Adults Only rating. We will see how this plays out when the inevitable exception to the poorly worded rule comes to pass.

Related Articles: 

ESRB Rates Grand Theft Auto IV

My Thoughts on the ESRB

ESRB Rates Two More PS1 Games For PSN