Without immigration and at current birth rates. According to the Population White Paper: A Sustainable Population for a Dynamic Singapore, we will have an inverted population structure by 2050. Singapore's citizen age profile will also shift.
Where as today we have relatively few citizens aged 65 and above compared to those below 65, this will change over the years as people live longer and each successive generation decreases in size. Eventually, we will have many more older Singaporeans than younger ones. Today, families generally have a few elderly members with a larger number in the younger generations. By 2030, this family structure is likely to reverse itself with more elderly members than younger ones. By 2050, Singapore will have an inverted population structure with more in the older age groups than the younger age groups.