I think so (speaking as a parent). I don't think parents should be ogres about the matter, but some monitoring is definitely a good thing. It comes down to the parents really knowing their children, and knowing what they could handle.
I was playing violent, M rated games at around age 6. I had (and still have) a very good relationship with my parents, we did things together often and spent a good bit of time together, so they knew very well how I responded to fiction, so they felt confident that letting me play games outside my age bracket wouldn't have any ill effects on me (to be fair, it was the PS1 era so it was hardly realistic and the vast majority of the enemies were aliens and monsters unlike today where it's normally human on human violence).
I think that if a child expresses an interest in a product (be it a book, movie or game) above their age bracket that it's up to the parents to know their child well enough to judge whether or not they're ready for the subject matter. Of course this also means the parent needs to do a little research to familiarize themselves with what the kid is interested in, and that seems like a lot to ask of some parents nowadays...
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Edit: Got it fixed. I had a rootkit that was disabling all Google functions, including captchas.