Brain maturation (fronto-cortical pruning) is not complete until 26 years of age.
Ever notice how you can work at 16, vote and be drafted/enlist at 18, drink at 21 but can't rent a car without a co-signer until 26?
Now you know why. Car rental agencies don't study human neurophysiology, but they damn well know there's a difference between people who rent their cars before and after 26.
I'm 65 and would happily take a hit for the team. I just retired from 30 years as a family doctor, and I see how people just start progressively losing their mental edge after 70. Not absolutely every single one, but most folks.
An alternative would be a test every voter takes to be able to vote. DMV-like 30-question multiple choice purely on facts of civics and current events. I don't see that happening given the history of poll tests in our country.
I would be happy with a voting age of 16, serving in public office at 26 and no voting or serving in public office at 70.
Sixteen year olds have the most skin in the game from the standpoint of having to live with the consequences of election outcomes. Sixteen year olds were allowed to vote in the Scottish independence referendum for exactly this reason.
Full brain maturation (fronto-cortical pruning) is not finalized until 26.
The incidence of cognitive impairment goes up significantly at 70 years old.
And also the long-term risks to your health. The likelihood of chronic back and knee pain as well as hearing loss is fairly obvious. However, there's also exposures to toxic chemicals in both open and closed environments that can put you at risk for cancers (especially lung, bone marrow, kidney and bladder) when you're older. It blows my mind that ALS (Lou Gehrig's disease) is unconditionally accepted as a service-connected condition. No one has any idea what exposures might be causing this, but the prevalence of it in former military people as opposed to civilians is so much higher that the VA just accepts it. It's and awful disease, untreatable (except nursing care) and incurable and the VA isn't going to have to cover care for long.