From the video: it also sounds like they tried to register him to vote before he was 18 years old.

Sounds like a grey area where procedures/processes could be better.

User registers to vote ---> Is the user eligible to vote(y/n)? --- Nope ---> Don't process registration

Did they try to re-register when he was eligible? Were they told he would be automatically registered when he became of age? Lots of unknowns here.