Typically you want complete coverage for a demographics question, but because this is after a screener, yes, you don't need the under 18 option for question 6.

It might be used an attention check, but it's also possible the survey author didn't think through the branching logic. When rushed, I've made similar mistakes.

It's one of the reasons you should always field test a survey first. Considering the other errors, it seems like they skipped this step.