Younger people vote less often in all elections, not just mid-terms. The Pew chart is interesting, but I'm not sure what the explanation is--age, cohort, historical moment, overall decline in mid-term participation, changes in eligibility rules--it gets pretty complicated. The chart does seem to show a generational effect, but I'd be curious about the methodology. Got a link? I think it would take a longitudinal study to give a definitive answer, but that takes generations to do. I'd be surprised if there's one that includes the Silent Generation.