Senou is 16. Age of majority in Japan is 20.

Madoka is mainly worried about being labeled a "shotacon", one who has a complex for young boys. Whether society would actually label her as such or if the thought is merely a stress-exacerbated paranoia on Madoka's part, who can say?

In case you missed it or forgot, Senou and Seiko are both half-American as their father is from the US.