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.
A former assistant US attorney, Andrew McCarthy, has spoken and Obama’s travels on any foreign passport from the time he turned 18 is VERY MUCH an admission of his allegiance to a foreign country and in doing so he automatically renounced his US citizenship.
PERIOD..no going back for Obama on that one!