Oops, sorry. He was 19 when his parents divorced in 1980, not 1970.
He attended Occidental in 1989-1990(then 28-29 years old), then transferred to Columbia in 1991 (at around 30 years old). It was not until the transfer to Columbia, that he changed his name from Barry Soetoro, to Barak Obama (Illegally). This means that well into adulthood, he kept the name Barry Soetoro, which also leads to the conclusion, since he did not get a US passport until he was a Jr Senator from Illinois, that he used an existing one, and I surmise, from Indonesia, which falls in line with being a foreign exchange student, traveling on a student visa from Indonesia (which kept is citizenship in tact) while in the states and attending Occidental, and his trip to Pakistan, and Indonesia to visit his mother and sister Maya.