Dating.
Yes it would be interesting to know more of the 'facts', as the girls actual age, although that would matter little as to the letter of the law.
I would wonder if, this 19 yr old man, had beend dating her and had been 'a guest' of the family. If he were visiting the girl at her home, say for sunday dinners e.g., where by he could show that her family had been aware, and encouraging to a relationship. could he not claim he had the parents' concent to a relationship?
It would 'show' the court that he took 'resonable care' to find out if the girl were honest of her age, and show diseption on the part of the parents as well, if not showing the parents were sanctioning the relationship; which may open up another legal issue.