As a US citizen, you are subject to US taxes on your income, regardless of your age and regardless of where you live or where your income is earned.
However, if this person lives and works overseas, and he is out of this country for at least 330 days in a 12 month consecutive period, then he may claim the Foreign Earned Income Exclusion. For the 2009 tax year, that amount is $91,400.
What that means that if he satisfies the 330 day requirement for being out of the country, then the first $91,400 he has in foreign earnings will not be subject to US income tax. He would still need to file a return with the IRS and report his income, but he would then claim the foreign earned income exclusion credit by filing Form 2555 with his return.
If he earns more than $91,400, he would owe tax on the excess earnings, but at the same time the US will allow him a credit for any tax he paid to Singapore on that same income, thereby eliminating any double taxation.
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