Three studies linking age structure (percentage of youth in the population) to civic outcomes. Conclusion: civic participation and civic knowledge patterns are transmitted socially, not just by immediately household.
Starting point: youth are more likely to volunteer, but have less civic knowledge (empirical finding for U.S. population).
Question: are young people who live in communities with more youth saturation even more likely to volunteer (and less likely to have civic knowledge) than young people who live in communities with more adults?
Answer: Yes. Even controlling for demographic factors and for characteristics of the survey respondents' parents (e.g., education, income, whether they volunteer, etc.)
This result also seems to hold up internationally: greater youth population correlates with more volunteering and less civic knowledge (even controlling for GDP).