Yes, her character and behavior is 100% irrelvant and because of rape shield laws, even if she had been sleeping with the football team, such evidence is inadmissable in court -- not only could the accused not use the information in his defense, the judge/jury would not even be allowed to see or hear about it. Rape shield laws prevent the victim's character from being called into question.
It's also irrelevant if she lied about her age. There are lots of people who went to jail for statutory rape who tried to tell the judge "But she told me she was 18. I even checked her ID, but I guess it was fake." Doesn't matter. If you had sexual relations with an underage person, you've committed a crime regardless of the circumstances or the victim's seeming "consent" or motive.
Really, the ONLY defense is if you can prove -- and under the rape shield laws it's VERY difficult to make this case -- that the victim is some sort of pathological liar or that she just said that the accused had sex with her to hurt him for some reason or because she was trying to get even with her boyfriend or some similar scenario or that her version of the events has inconsistencies and that he couldn't possibly have committed these acts at the time and place he was accused of doing so (and he has witnesses to that effect). If you can call into question whether the alleged sex act even occurred, then it may be possible to dismiss the charges on evidentiary grounds. But, if the sex act did happen, any motivation or "consent" or explanation is irrelevant and the accused will be convicted of statutory rape.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney