German was easily the second-most widely spoken language in the US after English up until the early 20th century, when most German-Americans concealed their ethnic ancestry during World War I and were more or less forced to completely extinguish it during World War II.
It makes sense that it would be more likely to survive in more homogenously German areas where there wouldn't have been as much public pressure not to speak German in public. But I'd imagine most of the Americans who speak German at home are over 65 and that their children and grandchildren speak English. German will die with them.
The same phenomenon is happening in Louisiana with Cajun French.
This indicates the cutoff is about 1950 (at least in New Braunfels).
http://www.tgdp.org/tgdp
