I don’t know. Take this scenario: 18 yr old dating a 17 yr old -> parents catch them in the act -> 18 yr old is charged with statutory rape -> 18 yr old is listed as a sex offender for the rest of his life.
18 yr old has to register as a sex offender online, tell his landlord and employer he’s a sex offender, move if he’s within X feet of a church or school or bus stop, fill out paperwork with local cops, never step foot in a church again even if he’s religious, stay far away from playgrounds, avoid kids in all situations, etc- all the while being labeled a sex offender, the same classification as a kiddie rapist. Is that just?
No, it’s not. But not because the guy is 18. A 17 yo. who consented to having sex with someone is either the victim of a sexual assault or she isn’t. The age of the “attacker” is irrelevant.
At 13 you’re just hitting puberty, and still in middle school if you live in the US. 16 seems much more reasonable to me.. but the whole question of ‘when are you mature enough to be able to consent?’ is something I don’t have an answer to. Still, I would say the age difference between participants is important. If the ‘rapist’ is 60 and has ‘consensual sex’ with his 13 year old piano student, it’s obvious that the 13 year old wasn’t mature enough to make that decision and was persuaded to do it. But if both participants are 16, [without knowing any other context], it seems more harmless.
Your scenario isn’t an argument in favor of age difference being considered. A valid scenario would be taking the same exact 13 yo. who was, in your scenario, incapable of consenting to sex with the 60 yo. piano teacher, and having her have sex with an 18 y.
Then, would it be logical to say that the same exact 13 yo. suddenly became more mature because of her partner’s age? Obviously not. The 18yo and the 60yo should face the same exact consequences for their wrong action.
Saying that the 18yo shouldn’t be treated the same way a rapist or a child predator is, is in fact also an argument for the 60yo not being treated that way.
P.S. I never suggested that having consensual sex with a 13-15 yo. should be legal, only that it shouldn’t be classified as sexual assault. A sexual assault presupposes that the victim is forced into sex, against their will. That is indeed true for a child, no matter if he/she agreed to it or not (children don’t have any understanding of the implications of consenting to sex, therefor they can’t make such a choice).
But it’s not true for a teenager. It still is a violation of rights to take advantage of a teenager’s immaturity in this way (just as it would be to take advantage of it in other ways, like having them sign a contract without their parents’ approval, confess to a crime, etc.) but it does not fit the essential characteristics of a sexual assault to do so. While their understanding of what they’re doing is not that of an adult, they do have a level of understanding. If they say yes, that is a meaningful yes, dismissing it and equating it with a “no” would be irrational.