And USA already disallows voting based on age. And somehow lot of people find it acceptable. Surely voting should be right from birth?
Also, surely you don't think a four year old should weigh in on who gets to represent them in Congress? We disallow voting on a minimum age, because children are still developing in all sorts of ways, there is no maximum age that takes away your right to vote when you reach it. You can't equate a minimum age to disenfranchisement.
And really why should not 4 year old get to decide? How come you think it is right to discriminate against them based on their age? As you said there is no limit on those with limited mental faculties due to old age. So why have one for young age? Isn't it clearly an unacceptable form disenfranchisement? Or are you saying some forms of that are absolutely acceptable and even justified?