Doesnt follow. Firstly rape in law is by a man, the scenario doesnt specify the sex of the 15 year olds.That's apples to oranges; just because a person can't be mature enough to consent to sex doesn't mean they can't be mature enough to rape someone.
Yes, I understand you're exploring the logic of this scenario - but the two aren't analogous. Both involve "consent", but the former doesn't involve a crime on the part of the consentee-in-question; the latter does. The issue is NOT whether people are old enough to consent to crime...
And the point is how can someone who cant consent be raped then? Rape by definition mrans they havent given consent.