Previously I asked What's the term for some sequel where we see the characters in the original but much older? where the answer is Character Aged with the Actor but in the examples I've mentioned the Motion Picture Association film rating or whatever doesn't change (except possibly Rugrats vs All Grown Up).
Is iCarly the 1st of its kind? Or is there precedence?
Note 1: When I say official, I mean to exclude not only 'Rule 34' or whatever and other fanfiction but also things like Once Upon a Time.
Note 2: As for the reverse case of kids' sequel to an adult series, I can't think of anything, but I don't imagine it's as rare. Maybe Back to the Future animated series. I really mean here adult sequel to kids' series.