When he was 21 and didn't drop his U.S. citizenship. Chapter VI, Section 91, Paragraph 1 of the Kenyan Constitution:
"A person who, upon the attainment of the age of twenty-one years, is a citizen of Kenya and also a citizen of some country other than Kenya shall, subject to subsection (7), cease to be a citizen of Kenya upon the specified date unless he has renounced his citizenship of that other country, taken the oath of allegiance and, in the case of a person who was born outside Kenya, made and registered such declaration of his intentions concerning residence as may be prescribed by or under an Act of Parliament."
Quibble all you want, your scenario concerning Obama is just as insane as my scenario concerning Romney.
Thanks.