DOS does not publish breakdowns of citizen population by age group. The published population pyramids are for “residents” which includes permanent residents. However, the government did publish the actual number of Singapore citizens aged 20-64 in 2011 in the population white paper together with forecasts up to 2060. Chart 6 shows their forecast for the number of citizens aged 20-64 in 2015 as around 2.2 million (I use their scenario with maximum intake of new citizens). The ratio of working-age to elderly citizens is around 5 in Chart 8. This allows us to estimate the total number of Singapore citizens 20 and above in 2015 as 2.64 million. An ELD press release on 27 July 2015 states that there are 2,460,977 electors on the electoral register. Thus, approximately 2.46/2.64 = 93% of Singaporeans of voting age are on the electoral register. For simplicity, I’m not adjusting the denominator for those above 20 but below 21.
Apart from those who were dropped from the the electoral register for failing to vote in previous elections, various other categories of people are not eligible to vote (Section 6 of the Parliamentary Election Act), but the biggest category are probably prisoners. The total number of prisoners (including drug detainees) in Singapore is around 10,000 so that should not affect the figure that much. I presume that the NPTD population figures do not include overseas Singaporeans, so that would not contribute to the missing 7% either. The predominant explanation for voting-age Singaporeans not included in the electoral register is probably be that they failed to vote in previous elections but did not apply to restore their names to the electoral register by choice, ignorance or inability to pay the $50 fee required. 93% registration is high by international standards, but does seem a bit low considering that Singaporeans are automatically enrolled when they turn 21 and only drop off (mostly) if they fail to vote without reasonable excuse.
The International Institute for Democracy and Electoral Assistance (International IDEA) has published a table showing that only about half of Singapore’s voting-age population actually voted. While the first three columns are consistent with ELD figures, the fifth column, Voting Age Population, appears to have been derived by subtracting the number of residents below 21 from the total population including non-citizens. The high proportion of non-citizens (including PRs) living in Singapore, especially in the working age population, therefore inflates the denominator and the low Voting-Age-Population turnout calculated by IDEA may be literally true but not meaningful because a large proportion of the denominator are non-citizens. The numerator is also only for voters in contested constituencies. The voter participation rate is thus further reduced by the prevalence of walkovers in past elections.
(Documenting here for reference as the question came up in discussion recently)