No, you don’t need to be an adult to play. You need to be an adult to create a Battlenet account and agree to a contract (the EULA). Parents accept that for their minor child that they allow to play.
As for the filter, no, that does not excuse people breaking the rules that they agreed to. Never has been permission to do so.
No, it really does not mean it is allowed. It exists because some people can’t follow the rules they agreed to. Going all the way back to 2004. This post is from 2006.
EDIT: I see nobully already posted that example. Ah well. Leaving it in there.
I will give another more recent one Spotlight on: Harassment
Mature Language Filter (aka The Profanity Filter)
This can be activated within your Interface Options: Main menu (ESC key by default) Click Interface > Social and check - Mature Language Filter.
Once enabled, all inappropriate words in our profanity database will be filtered and masked to appear as jumbled characters, such as ‘*##@&’.
Note: The filter does not excuse the language used. The filter serves as a temporary shield, to help parents of minors and others who do not wish to see it, to block it. Our policies prohibit the use of both clear and masked inappropriate language.