Well, yas. The typical eight-year-old does not have enough knowledge or life experience to give informed consent. Typically, a child would not have any idea that she might be risking syphilis or something worse. An eight-year-old wouldn't even know what syphilis is. And a hypothetical extremely precocious eight-year-old who had all the medical knowledge about such matters in all likelihood *still* wouldn't have the judgment to decide wisely, nor the life experience to judge whether or not her sweaty bald fat fifty-year-old would-be partner is likely to dismember her afterwards and stuff her tiny corpse into the crawl space of his house--a non-trivial risk, when you consider just how fucked up a man would have to be to want to fuck an eight-year-old girl.
Sex is an adult matter, for adults. This isn't just a tautology.