She wasn't a minor, she was 18. However the law required that she not only be a citizen, but also have lived in the country for a certain number of years after a certain age, and she failed to meet that criteria. So he was not a citizen when he was born. That's pretty much the minimum to be a "Natural Born Citizen".
For those who might have missed it:
Obama Born in Kenya? [Saturday Update in Texasdarlin]
http://www.freerepublic.com/focus/bloggers/2103299/posts