To clear the doubts of anyone in future looking at this thread
for answers: 
The straight-forward answer: YES, it is legal as long as it is
16 and above and with the consent of the party.
According to Singapore Law, Chapter 224 - Penal Code, Section
367A, subsection 1, it is an offense to engage in sexual activities
with a minor (regardless male or female) below age 16. Sexual
activities includes anal and oral.
One interesting thing to note is that according to Singapore
Law, Chapter 224 - Penal Code, Section 367A, subsection 5, it is
perfectly legal for a male to penetrate his wife's vagina with his
penis with or without consent of his wife as long as his wife is
not under the age of 13.
This means that you can have sex with a girl with or without
permission who's age is under 16 but above 12 only if the girl is
legally married to you and are only allowed to vagina penetration
since the law did not states oral and anal are allowed.
However, subsection 5 is not without limitation. The subsection
5 does not applies, for example, if his wife is living apart from
him due to:
Full/More information on the limitation of subsection 5 can be
found at: http://statutes.agc.gov.sg/
I take no responsibility for any wrong interpretation made by me
but definitely the minimum age is 16 to have sex legally. Feel free
to correct me if I interpret it wrongly.
you got check all sources and latest updated ones or not. If not
anal and oral what aboutn intercourse? This is very important
unless anal also include intercourse? Just state penetration lah
the law like that will cover anal or iuntecourse. or maye just use
oral which will cover all actions which is not fatasy. so
fantasizeing this type of act is legal loh.