An illiterate or blind adult can ask for help. A poll worker will read the ballot or provide a braille version to help them, and will fill out the ballot with them if requested.

I'm still agreeing with you, you've convinced me that any age barrier is arbitrary and hypocritical.

As for registration being a hurdle, the courts have long held that the effort to be registered is minimal, as again there are resources to help people get registered. There are outreach programs, and you can actually go to your local post office or dmv and they will help you register. Children would have an even easier time, since anyone in school could have a teacher or school staff help them.

Children are particularly beholden to their parents for support, though, and by "certain type" I mean the type of person who thinks that having a child is a means to an end. There's a whole spectrum of quality parenting decisions, but as a general rule, anyone who is having more kids to have more votes is probably a bad parent.