Being legally adopted in Indonesia (which there is no evidence of happening) would not require him to file for US citizenship at 18.
No actions by your parents can cost you your citizenship if born in the USA. You would have to take a pro-active step, like refusing to live in the USA, voting as the citizen of another country, etc. Something that actively shows you have accepted citizenship elsewhere is required. Without it, your USA citizenship stands.
If you have evidence that Obama served in the Indonesian Army, or voted in their elections, you might be on to something...
Like renewing your passport to travel to Indonesia and Pakistan? Not saying he did, this is just conjecture. IF he did, that would reaffirm his citizenship to Indonesia.
Other than the divorce papers between his mother and Lolo Soertero. Those describe him as a child of the marriage. Since we know, from the divorce papers between his mother and Barack H. Obama Sr, that he was also child of that marriage. We must assume that he was legally Soetero's child, via adoption, albeit one over 18 but still needing support for college, we pretty well must assume that he was adopted by Soertero. Might have been in Hawaii, or Indonesia, or elsewhere for that matter.