I don't practice in Iowa. Generally, yes. If there is a curfew, your 19 yo contributed to their staying out past it and breaking the law, then he can be charged.
Edward J. Blum
Like Mr. Blum, I do not practice in IA, but I'm not sure I can agree with him. "Curfew," at least in IL (where I practice) is not an act of delinquency but is only a status offense. Thus, in my jurisdiction I don't see how one could be convicted of contributing to delinquency when the offense itself could not result in delinquency proceedings. That being said, the prosecutors can charge just about whatever they want; what they can prove beyond a reasonable doubt is a whole different story. I suggest you tap your yellow pages or search engine to ask your question specifically to an attorney in the county where your son is being prosecuted. Many lawyers will grant a free initial consultation.