My personal belief is that if you want to claim that consent was not able to be given, you have to prove so. You know, innocent until proven guilty. Proof may include medical data indicating the person hasn't reached puberty, psychological examination indicating a lack of understanding about sex and sexual issues, mental illnesses, a severe abuse of a trusted position (preacher, teacher, etc), and so on. Don't limit it to one thing, let the prosecution make a case, and let the jury decide if its valid. Whether someone is competent to consent is so arbitrary, that any hard and fast rule that doesn't allow gray areas is going to result in some very bad decisions.
I don't think that this would hinder the prosecution of the real predators out there, but would certainly make it a lot harder to prosecute those indiscretions between two teenagers, or some jail-bait with a fake ID.
As far as pornography, I think they should also have to prove that you sought it out, and were consuming significant amounts of it, and/or distributing it. Just clicking on the wrong link, and getting some in your browser cache shouldn't be enough to railroad you into jail. Again, they should have to make a case that you are encouraging the distribution or production of it, and thus encouraging illegal acts. None of this instantly-guilty bullshit.