I don't know when the Joe Foss Institute performed the test, but as of Jan. 1, 2017 Tennessee is requiring "all of the state’s high school students to take a civics test like those administered to immigrants during the naturalization process."

The students are not required to pass the test.

That test illustrates what I dislike most about typical history tests. They are more along the line of "When was the Magna Carta signed?" instead of "What was the major significance of the Magna Carta?"

Most people won't know either answer but the latter has relevance today, whereas the former is merely a trivia trinket.

I missed my calling. I should have been an immigrant.

I would agree with your assumption - "However, I am a little surprised about the score gap between the ages. I thought if you were young you would remember more what you learned in school than the elders."

The fallacy is that the young are not taught the same.