> There is much less variance on cognitive ability on the younger end of our lives than the higher end. I'm not sure that's true. Mental maturity in kids varie

> There is much less variance on cognitive ability on the younger end of our lives than the higher end. I'm not sure that's true. Mental maturity in kids varies widely, too. There are some pretty mentally mature kids changing the world. Look at Greta Thunberg. There are also many fewer years for older people to live with the consequences of their votes. Everything has trade-offs. In this case, the loss would not be as bad as the loss we get from excluding those below 18. Whatever the counterexample of sharp old people, I can think of one for sharp young people. We ignore those exceptions for young people, we can ignore them for old people, too. And if you're worried the old-age cutoff will be too young, just pick whatever age's median mental acuity matches 18 year olds - what's good for the gosling is good for the goose.