At this point, when completing registration, new board users are automatically placed in usergroup Members.
There is no setting to make variations in that like based on date of birth or location, where that could come in handy when you want to create a multi language board, but don't want to show the forums in other languages, or when you want specific rights to be given to people of specific ages like a teens area, an erotic area etc.

It would be great if such a feature was available in a future release (preferably 3.0.4, since 3.0.3 is allready released)

Something like this wouldn't be introduced in 3.0.4. It would wait until a major update (like 3.1) if it were to be introduced.