258m adults in the US in 2020 about 255m in 2016
74 223 975 votes in 2020, 62 984 828 in 2016
That's both more than 18% and there's no reason to believe that all people under 18 would vote against Trump.
And that's only based on the people that bothered voting, there's supporters that didn't vote that should get added to that as well.