I try to respect everyone, until they have given me reasons not to. That includes 5 year olds, 10 year olds, and 20 year olds. But the OP was talking about 18 to 40 year olds, and I don’t see any reason to disrespect members of that that group.
Also, I will not most of the lack of respect for “older people” the OP noted came from those of us in that demographic.
I just want to make sure I’m reading this correctly: in, for example, the 2000 election, Gore famously lost due to a mere 537-vote margin in Florida — which means, as far as I can tell, that if a mere 269 voters thereabouts had switched their votes from Bush to Gore, then, uh, insert folksy phrase here, right?
My father was a teenager when he was serving in the military; and, as I understand it, he got to cast a vote on the big important question of who should be the next Commander-in-Chief bossing around a bunch of conscripts who have to get funny haircuts and wear target uniforms. And, well, I can’t imagine figuring my dad shouldn’t have gotten that say.
And as for me, I was all of six years old the first time I heard the lyric about being old enough to kill but not for voting — and I’m old enough these days to satisfy the OP’s requirements, but it sure wasn’t hard to reach the same conclusion then that I’m still on about now.
Older people were more likely to vote for Trump, which proves their wisdom.
Non-college educated were more likely to vote for Trump, which proves folk wisdom is more important than qualifications and book-learnin’.
I think the voting age should be lowered to encourage participation. I think the drinking age should be about the same everywhere. I think your maximum driving speed (in kilometres per hour) should be three times your age - because younger drivers lack experience and older drivers… well, maybe I can’t justify it, but that’s only because it isn’t a good idea.
Evidence suggests that the human brain is not fully developed until sometime after age 74. So drinking, driving, voting, serving in the military and especially holding high elected office should not be allowed until some age greater than that.
I seem to recall that your voter registration lapses in some places if you don’t vote for a certain amount of cycles. So that is already being done, just not to the extant of felony disenfranchisement.