I immigrated to Europe Malta when I was 9 years old that wS 21 years ago now. I never worked in the states I am planning in moving back to states in October. Am I exempt in filing tax returns?

You haven't worked in the US, so no, you don't have to file taxes for all of those years you weren't there. As soon as you get a job in the US, and living there, then you start filing income tax.

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A U.S. citizen or resident must report worldwide income even when he or she does not live in the U.S. If your total worldwide income exceeds the filing requirement, a U.S. citizen or resident must file your U.S. tax return. If you paid taxes in the foreign country, you will claim credit for taxes paid in the foreign country by filing Form 1116 or can file form 2555 Foreign Earned Income Exclusion. Even if you have a filing requirement, the chances are you will not pay any U.S. tax or pay very little U.S. tax.
Another filing requirement is Form TD F 90-22.1. Your U.S. Tax Return: U.S. Citizen or Resident with Foreign Income (http://taxipay.blogspot.com/2008/03/us-citizen-or-resident-with-foreign.html)

If you are a US citizen you need to file a tax return even if you do not live in the US as long as your income is above the limit. You can exclude a certain amount from being taxed on form 2555.

You can see the limits here:
http://turbotax.intuit.com/support/iq/Other-Tax-Situations/Do-I-Need-to-File-a-2010-Income-Tax-Return-/GEN12413.html