How Do Exemptions Work When Moving Back to US?

I have been living outside of the US for around 2.5 years, but plan on moving back later this year. It is absolutely necessary for me to sign up for insurance there? I do not know where I will live yet, so how will I know where to sign up for the insurance?

I read about a fee for those who didn’t sign up within the enrollment period, but since I’m not currently living in the US, will the fee affect me? I will file taxes for 2014.

I have health insurance here, does that affect or count for anything?

I also turned 26 this February, so based on my research I still can sign up for the marketplace coverage, is that right? And I qualify for the special enrollment, correct?

I look forward to your help!