Depends what ratings are intended to do - if they are just to be informative for potential viewers then the kind of system you propose sounds good.

But, if the main purpose is to legally restrict access based on age, such ratings wouldn't really do the job.

Plus, of course, there's quite a range within each category - language could mean something as innocuous as "bum", or something as deeply offensive as "Belgium" or "Barbara Streisand".

I suppose you could have age ratings for each subcategory - L18, V12, N0, S0, D15, but it'd start to get complicated pretty fast. Plus, presumably, whatever the highest of those ratings was would still be the overall rating. At which point we're back where we started.