I believe each State has differing laws regarding statutes of limitation. Not sure which State would have jurisdiction.
Regarding rape charges in general there are many old rape kits that can now be tested with current dna technology. This is the best reason not to impose a statute of limitation regarding rape.
Regarding statutory rape that occurred 50 years ago it would seem moot for the victim to now object. Not filing a complaint in a timely manner should be grounds for dismissal.
