I do agree that there is probably no perfect answer, and that we haven’t even touched on the issues with the perception of appropriate age outside the US (predominantly) and a word to the UK.
If I was the one making the decisions, I would probably set the age of informed choice at 18. It is an age where just about everyone agrees you’re old enough for the majority of important decisions in your life. Not perfect, but just said nothing is going to be.
Having said that, I would probably put the age of partial liability at 15, being at least vaguely realistic in what teens are like too, wherein consensual sex in a 3 year gap (15-18) is a misdemeanor crime punishable by community service. Not going to really affect a young person’s future, but something that makes them take it seriously. And that’s assuming someone in the immediate group even reports it (likely parents in this case). I do figure that is the best balance I can between comparative sexual and mental maturity. Anything past (15 and a 19) I’m going to come down on too much a power and experience gap.
Anything younger than 15, I’m sorry, I can’t see as anything other than a crime - no matter the hormones and physical maturity. It may not be rape as we consider it now, very possibly the young partner is curious/willing/etc, but I cannot think they have anywhere close to sufficient maturity to make any sort of such decision. Perhaps we could try another label than ‘rape’, which is of course heavily loaded, but I still consider it a crime. How it would be punished would again vary per the age of the other party. Very extreme IMHO if the other party is 18 and older, and perhaps less so under, but far more than what I suggest in the age group above.
ETA - And in case I wasn’t clear, if it isn’t clear minded, verbally consensual then it’s rape. Period. End of Sentence. I think everyone here is clear on that in the thread though.