My first thought was that it was one of his opponent's kids. Seems to often work that way. His opponent is a 54 year old Republican, which makes it unlikely that he has any kids in the 17-22 range. At this point, I'd be looking at the kids of his opponent's campaign staff.
Basically, you're probably looking at a younger person, and people of that age don't tend to be so riled up about party politics without some sort of direct involvement, like a familial association.