He can NOT POSSIBLY be a "natural born citizen" of the US because his father, Barack Hussein Obama Senior, was a Citizen of UK and/or Kenya.
His mother was a citizen. Nobody doubts that. Why doesn't that count?
It counts but not when the mother is under 18. After becoming 18 she could have passed on citizenship to her offspring.
Rep. John Bingham of Ohio, considered the father of the Fourteenth Amendment, confirms the understanding and construction the framers used in regards to birthright and jurisdiction while speaking on civil rights of citizens in the House on March 9, 1866:
... I find no fault with the introductory clause [S 61 Bill], which is simply declaratory of what is written in the Constitution, that every human being born within the jurisdiction of the United States of parents not owing allegiance to any foreign sovereignty is, in the language of your Constitution itself, a natural born citizen...