Despite the hype around young Americans' civic activism and record voter turnout in 2018, the voting power of young people is shrinking.

The big picture: On top of young adults being less likely to show up at the polls, the number of people under 25 who are even eligible to vote has fallen, according to a Census data analysis by Brookings Institution's William Frey.

By the numbers: The 65+ year olds' share of the electorate is projected to be 23% in 2020, up from 18% in 2000, Pew Research's Richard Fry told Axios. Meanwhile, the share of eligible voters who are 18-24 is expected to fall one percentage point to 12% in 2020.

Gen Z is a much smaller generation compared to the baby boomers (ages 55–73) or millennials (23–38) — who are typically the kids of boomers.

Even millennials' voting power is weakened by a higher share (12%) of non-citizens who are ineligible to vote, according to Frey.

Not only outnumbered, young people today are also outvoted by older generations.

71% of eligible 65+ year-olds, and 67% of 45–64 year-olds, voted in 2016. Less than half of 18–29 year-olds did, according to census data.

Why it matters: Steady, high turnout among older Americans could help President Trump and hurt Democratic candidates in 2020.

The older population is whiter and more conservative than younger Americans.

Trump's 2016 election relied heavily on that demographic.

The Trump re-election campaign is spending significantly more money than any other candidate targeting seniors on Facebook.

The other side: Gen Z and millennials have been sticking with Democrats even as they get older, which does not bode well for the GOP in the long-term.

"Anyone in the sort of 18 to 45 age range is basically voting 2-to-1 for Democrats over Republicans," Brian Schaffner, a political scientist at Tufts University, told Axios.

What to watch: If young people are energized by a candidate they can make all the difference, says Stillman, pointing to young voters' support for Barack Obama in 2008.

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