Exactly. The concept behind statutory rape laws is that a 13-year-old is fundamentally incapable of consenting to sex. As soon as one says, "... except if the other party is (fill in the blank)," then one is contending that a 13-year-old is fundamentally capable of consenting to sex, and all rules other than "not by physical force" can be considered arbitrary restrictions on liberty.
/johnny
Then s/he is also fundamentally incapable of having criminal intent and cannot be charged with a sex crime. You can’t have it both ways.
And, BTW, many countries and states disagree with your stated assumption of the purpose of these laws. That is why they do make a distinction according the ages of the parties involved.
I imagine that if one is, by definition, incapable of giving legally-valid consent to sex, then one is, by definition, incapable of forming legally-recognizable culpable intent to have sex. Thus, neither child could be charged with a crime, if intent has any meaning in the process. I'm not fond of criminal laws that dispense with the need that the “criminal” actually have some sort of culpable intent.
So, to me, the obvious solution is that neither party could be held criminally liable for statutory rape (or its equivalent), as neither had the capacity to form a legally-valid intention to have sex.
If force had been involved, if one party had forced the other, one could charge the offending party with some sort of assault, since there is nothing in the law that suggests that minors can't form the intent to commit acts of violence.
Just my two cents.
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