Children can not consent to sex...we don't think they have the judgement to drive, open bank accounts, vote etc. We don't let them decide whether to go to school or not. But some idiots think they can consent to sex? They believe that to justify what they do and for no other logical reason. In fact

Under most laws, young people are recognized as adults at age 18. But emerging science about brain development suggests that most people don't reach full maturity until the age 25. ..... So the changes that happen between 18 and 25 are a continuation of the process that starts around puberty, and 18 year olds are about halfway through that process. Their prefrontal cortex is not yet fully developed. That's the part of the brain that helps you to inhibit impulses and to plan and organize your behavior to reach a goal.

And the other part of the brain that is different in adolescence is that the brain's reward system becomes highly active right around the time of puberty and then gradually goes back to an adult level, which it reaches around age 25 and that makes adolescents and young adults more interested in entering uncertain situations to seek out and try to find whether there might be a possibility of gaining something from those situations.

http://www.npr.org/templates/story/story.php?storyId=141164708

Worth reading the whole article to get some scientific backing for our intuitive sense that children cannot make informed judgement about having sex. And of course where a child is along that age spectrum is important. How could a pre-verbal child give consent to sex.