So, during the night before her 18th birthday, she's given the sound mind of an adult and can then go seek to abort her baby? I'm missing something from your post.
That's the way the law works, yes. It's because they have to apply to 300 million people with 300 million sets of life experiences, so they find an arbitrary line and stick to it on principle. What's the alternative, having judges decide when someone is old enough to vote as an individual?