Take the victims age, subtract 10, double it and add 11. Anyone that age or older is prosecuted. Add 13 instead ... anyone that age or older is a registered sex offender. Add 15 instead ... anyone that age or older is executed.
Well, apart from the death penalty part I think this is an interesting approach.
And right away it also becomes clear how hard it is to fix a reasonable, coherent and consistent approach in words.
For example, in your approach a 17 year old isn't considered mature enough to consent to sex with a 25 year old, and at the same time the death penalty is considered a proper punishment for other (or maybe even the very same!) 17 year olds.
I accept that your proposal is just a quick (yet interesting!) example. I'm also not pinning you down on it. Just pointing out how hard it is to put reasonableness into rules.
And in the end I guess we all want the same: protect the vulnerable from abuse, and yet not victimize those who who aren't abusers. It's jsut that we need to find ways to agree on how to do that.