I believe it to be a fact that even people who have been diagnosed as insane and incompetent

[Ms. Toad]

32. We have set the age of voting at 18 in this country.

That means anyone who meets the criteria is eligible to vote unless disenfranchised - and since the right to vote is a constitutional right, that is pretty tough to do. That is an equal protection issue - treating some individuals (those with intellectual disabilities or mental illness) differently from all other all individuals similarly situated (within the age of the voting franchise). To deprive individuals of a fundamental right (voting) based on class membership has to pass strict scrutiny - essentially it has to be a necessary class distinction to achieve a compelling government interest. The most notable class distinction for disenfranchisement is conviction of a fenony.

Disenfranchising someone who is otherwise eligible to vote based on a class distinction is a very different question than expanding the franchise to younger individuals.