Yeah, that's why I created my own "Age-Rating System" when other people won't review/rate my games (like Common-Sense Media, ESRB, Pegi, etc.) because they aren't big enough or I don't have the money.
Again, I don't thing a required feature (at least from where I live) but, it would definitely be helpful
Also, I think you miss understood me, it wouldn't be automatic, the developer would select the age(-range) and parameters, maybe their could be an automatic option so that, depending on the parameters set by developers, it would select an age range.
Like a rating of 'Nudity' would set the minimum age to 17, 18, or something.
It could even detect where the user (player) is from and change parameters, so the default age would be 8+ or something, and each rating would change that number (Basic Examples: Nudity adds 10, Bloodshed adds 4, Blood & Gore adds 6, etc. and with a max number of 19 or somewhere around that), these changes would be different for other countries based on their standards, but that would turn this from a Basic Age-Rating System to an Advanced Age-Rating Algorithm.