http://www.ica.gov.sg/page.aspx?pageid=132&secid=130
A person who is at least 21 years of age and has been a Singapore Permanent Resident (SPR) for at least 2 to 6 years prior to the date of application. An applicant who wishes to apply on his/her own merits may come for a preliminary assessment interview after 1 year of permanent residence.