If the answer above is None it means there is no specific mention in the legislation, but you still need to ask local law enforcement before drinking in Singapore.
It is prohibited to sell alcohol to minors. Anyone caught selling, furnishing or acquiring alcohol in any licensed premise to persons below 18 can be fined up to $10,000. It is also prohibited for minors to purchase, or attempt to purchase alcohol from any licensed premises, in which the minors can also be fined up to $10,000. However, the authorities rarely enforced this on minors. It is technically legal for minors to possess and consume alcohol at home and in public (not in any licensed premises) as there is no law prohibiting it. It is also technically legal for someone to purchase alcohol and pass it to minors outside the store or licensed premise.
You can find more legal details and links here:
Wikipedia Legal Drinking Page