The 21 years old thing is abit rubbish lah. There is a United
Nations International Treaty from 1949 regarding rules on
international driving.
This treaty makes International License valid in foreign countries,
part of the requirements is that the holder must be at least 18
years old.
If singapore unilaterally make this age 21, they will give problems
to foreigners driving in Singapore using international
license.
Anyway, to answer the threadstarter's question. You can convert
your license in Singapore provide it has not expired.
Depending where your license is issued, that jurisdiction might
require you to prove that you have driven during this 6 years the
license had lapsed.
For singapore side, they want to see a drivers record, your valid
license, and letters stating your legitimate reasons for staying
overseas. And of course, pass your Basic Theory in SIngapore.
