Oath for citizenship acquired as a minor
A person who acquired citizenship as a minor by descent or registration must take the Oath of Renunciation, Allegiance and Loyalty after turning 21 and before turning 22; failure results in loss of citizenship at 22. Prepare the identity card and foreign-citizenship renunciation certificate. The service is free and normally completed the same day. Persons overseas may approach the Singapore mission.
Official ICA source
Written independently from ICA’s English original. The linked source supplies the complete document and subsequent updates.