The 26th Amendment was ratified in July 1971 in which the drinking age and voting age were lowered to 18, thus allowing young military to vote. This means that Obama would have had to trot himself down to the local immigration office in 1979 and renounce his Indonesian citizenship unless his mother had already done that upon Obi's return at age 11, but this is highly unlikely due to the fac that he is named as an of age child dependent on monetary support in the Dunham/Soetoro divorce papers.
Obama’s actions from the day he turned 18(legal adult in 1979) rest soley on him and since the divorce was not final until 1980 and Barry was already enrolled in Occidental, it is highly likely that he was registered there as an Indonesian student and therefore he, himself declared loyalty to Indonesia instead of the USA.
But, this still does not exclude the fact he was British at birth and therefore at best was only a ‘native’ citizen born on soil to only 1 American parent per the 14th amendment.
Depending on what soil he was born on.
Legal Adult is not the same for all purposes, AFAIK, it was still 21 for renouncing your citizenship. But he did not need to renounce any Indonesian or Kenyan or British citizenship in order to retain his US Citizenship, assuming he had it to begin with, or was naturalized along the way somewhere. Same, without the naturalization obviously, with Natural born citizenship.
Basically US law does not care what other countries might think on the issue. However traveling on a foreign passport, when entering the US, after whatever the "renunciation age" was at the time, is one method of renunciation.