The U.S. Census Bureau is out today with new data on young-adult voting trends in presidential elections since 1964. And though the research finds an uptick in recent young voter engagement, there's still a significant gap between young-adult participation through voting when compared with older age groups.

There's perhaps no better evidence of this than in the disparity between each age group's share of the eligible population in the U.S. compared with the percentage of the age groups that vote.

Here's the chart from the Census:

Census voting chart U.S. Census Bureau

In 2012, the most recent presidential election, people aged 18-29 made up only 15.4% of the eligible voting population. However, 18- to 29-year-olds account for 21.2% of the population — a difference of 5.8% reflected in the chart. 

Older age groups, by contrast, account for a much larger share of the voting population than they represent overall. People aged 45-64, for example, account for 35.6% of the eligible population. But they represented 39.1% of the 2012 voting population. Those aged 65 and older made up 19.1% of the eligible population. But they, too, over-represented their age group at the ballot box — 22.3%.

"Overall, younger Americans have consistently under-voted at the polls relative to their eligibility. The magnitude of these differences has fluctuated over time," writes Thom File, the author of the study.