Update: Turns out Wikipedia publishes the age of consent for every state in the US. For that information, and much more that might be useful in making the argument, go here.
I've previously written a post -- which can be found Do 17 Year Olds Have a Constitutional Right to Intercourse -- on the constitutional right of 17 year olds to have intercourse. It's not as frivolous an argument as you might think, rooted in the analysis in Lawrence v. Texas, and the fact that the overwhelming majority of states have an age of consent 17 or less. (Again, what does Washington or Iowa know that Oregon doesn't?)
The question is, can a defendant assert the 17 year old's right to intercourse?
Normally, a defendant cannot assert the rights of another person. This rule does not apply when two criteria are met, and both are met in this case.
We have not treated this rule as absolute, however, recognizing that there may be circumstances where it is necessary to grant a third party standing to assert the rights of another. But we have limited this exception by requiring that a party seeking third-party standing make two additional showings. First, we have asked whether the party asserting the right has a "close" relationship with the person who possesses the right. Powers v. Ohio, 499 U.S. 400, 411, 113 L. Ed. 2d 411, 111 S. Ct. 1364 (1991). Second, we have considered whether there is a "hindrance" to the possessor's ability to protect his own interests. Ibid.
Kowalski v. Tesmer, 543 US 125, 129-130 (2004)
It would seem that both of those criteria are pretty easily met. Relationships don't get much "closer," and there's no vehicle for the 17 year old to assert that right herself.
Anyway, this is something to add to the arsenal when the state charges your 21 year old client with Sex Abuse II -- a felony automatically requiring sex offender registration -- for sex with a 17 year old.