Why should the applications get to decide if they are appropriate for a particular age? Shouldn't that be up to the parent? I shouldn't need to tell my kid: "Well, to use this compiler software, you need to set your age to 18 temporarily, because some product manager 3,000 miles away decided to rate it 18+. But, set it back to age 13 afterwards because you shouldn't be on adult sites." It's stupid.

That is what I meant by age(-rating), you are correct. However, drop country specifics - too complicated. Age brackets are enough: child, preteen, teen, adult. At around 16-17 these should be dropped anyway since at that point people are smart enough to get around these measures anyway and usually have non-parent controlled devices.