It doesn't matter if you have a foreign license and IDP.
it is the LAW that no one under the age of 18 will drive in
Singapore.
So even if you are fully licensed by other countries you are still
flouting local law.
More over it is by international treaty that IDP is only issued by
people over the age of 18.
if anybody under the age of 18 is issued with IDP then it is issued
by 'non-compentant' authorities (i.e. one that is not authorised by
that countries government to issue IDP)