Most people of any ideology uphold some idea of an "age of consent": an age before which a person is supposedly unable to consent to certain types of interaction and those acts should be prevented even if the "victim" wants to do them.
I've had this argument a lot, and the main explanation I get is "a child can't consent to sex because they can't understand the implications of that act". If that's how consent works then I can also argue:
You don't actually consent to buying product X, you're just too economically illiterate to know that it's not good for you. It should be illegal and the people who try to sell it to you should be punished to protect you.
Recreational drug use should be illegal because it causes medical issues and most of the people who use them only do so out of ignorance of the risks, so they aren't really consenting;
Unhealthy eating should be illegal because it causes medical issues and most of the people who do it only do so out of ignorance of the risks, so they aren't really consenting;
Kids wasting their childhood on games (like I did mine) should be illegal because they only do it because they don't know they'll regret it, so they're not really consenting to playing games;
Learning Haskell as your first programming language should be illegal because it's a bad choice and you'd oly do if you didn't know that, in which case you're not really consenting. It should be legally required to learn something like Python or Go first. This constitutes protecting programmers' consent!
Obviously that's not how consent works. Consent is only invalid if it's obtained through deception or threats; just not having relevant information doesn't invalidate it.
I find it telling that all the people I argue with about this draw the magic line at 18. I guess it's just a coincidence that that's the number chosen by the government that oppresses them? But they don't always draw it there - sometimes, when talking about a case that happened in a state where the legal age was 17, the same person who normally draws it at 18 thinks it's okay because 17 year olds aren't kids. And these are mostly Ancaps I'm talking about! (You know, the same people who have no problem with prostitution?)
Be honest. If you believe in this, are you sure you're not getting it from the government? What do you think of past societies where marriage long before 18 was normal? Were all of those people rapists?
I also want to raise the spectrum problem: if it were actually about maturity, then not only would it not be bound to a constant age, but it couldn't be a hard line because maturity doesn't work that way.
A common response is to admit that the age chosen is arbitrary, but say we have to choose one as a practical matter (because god forbid we have to actually consider the individuals involved). But when you respond to an argument about legitimacy with an argument about practicality, the most you can be arguing for is that having a hard line age of consent is an evil means justified by its results - otherwise, you'd be committing the appeal to consequences fallacy. And that's not how any of the people I argue with see it.
The irony here is that it's actually the enforcement of an "age of consent" that violates children's consent, since it initiates force to take away their choice.
Now don't get me wrong, I'm not running some kind of apologism for child traffickers. Deliberate deception is equivalent to force. That includes omitting information you know you're expected to give, and children generally trust adults to an extreme. Children can also perceive implicit threats in this kind of situation which obviously invalidates consent. I'm sure that in practice, most pedophilic relationships are coercive, and I'll even join in socially punishing the perpetrators of even the ones that aren't. But it's not like it's impossible for any child to consent under any circumstances.
But I think this is actually a really serious issue because of how far it's taken. It's not just that people are wrong about ethics. Almost every Ancap I know thinks people who have sex with a willing 17 year old (in a state where it's illegal) should be killed. It's unbelievable how unanimous it is, and how against voluntaryist principles. You can post horrific memes about feeding people into woodchippers for saying "MAP rights" in Ancap communities and you will never get a negative reaction.
And you don't even have to be a pedophile yourself to get the woodchipper! Every time I make this argument on Twitter (which has happened a lot), I get several people talking about killing me. Once I even said as much to someone else, but I described as "joking about killing me", and multiple of them corrected me that it's not a joke. I think they would really kill me just for believing this.
It's terrifying. Are Ancaps actually going to murder a whole bunch of people when we finally get that voluntary world?