Most people of any political or 'religious' faction 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 if they say they consent then they're wrong, and the people who believe in this will therefore claim that the act must be prevented even if both parties explicitly say they consent.
I've never heard any such person attempt to define consent, but beyond my doubt that they'd define in it a way that doesn't leave the age of consent an obviously made-up special case, the idea falls apart intrinsically.
Suppose a child below the arbitrary age of consent is in a relationship which they are judged to be incapable of consenting to, but they declare that they consent and it's clear there's no coercion from the other party. The philosophy of the age of consent here is "you can't consent, you're too young" but what does that entail doing about it? It entails preventing the action by force. In other words, forcing a decision on the child about their voluntary interactions with others. This concept, ostensibly invented to protect children from manipulation, can actually only work by violating children's consent. (Damn it's almost like I've heard this before.)
Consent that is actually uninformed is not consent. Thus asking a five-year-old "do you consent to me shooting you" and getting a nod doesn't justify the act because the kid doesn't doesn't know the impliciations of death and explaining it to him in our terms will not imbue that understanding.
An argument's likely to be made to the effect of "a child consenting to sex counts as uninformed consent because they can't understand the implications of that act". That's not what uninformed consent means. If that were the definition of uninformed consent then I could 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 even if you want to buy it.
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'll deeply regret it for the rest of their life and they only do it because they don't realize that, so they're not really consenting to being allowed to play games;
Learning Haskell as your first programming language should be illegal because it will result in you getting frustrated and quitting programming and the only way you'd try to learn Haskell first is if you didn't know about 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!
It's obvious that uninformed consent is supposed to mean lacking physical information, not just being unaware that you'll end up regretting it.
Even besides that this argument can't work when you try to apply it to sex because sex is something that derives its specialness (apart from procreation which we'll assume isn't relevant here) from people feeling that it is special. If a person doesn't understand the meaning of sex, then to them it hasn't that meaning and it won't gain it retroactively.
If someone presents credible scientific evidence that such action is physically harmful regardless, that still would only be valid insofar as the child doesn't know about this research, but the on-principle 'age of consent' fake idea is not legitimate whatsoever.