A)Yes, your immortal.
B)No, your not Edward Cullen.
C)There are no downsides, except for getting fat if you eat a lot (still not going to get you killed though).
D) The fact that you can't get sick... is that a super power? No health issues (eg. Cancer, Tumors, Other deadly diseases).
E)Let's make it 21.. because an average person is at their best physical shape during that time I think?
Thanks for the clarification.
First thing's first. I'd cancel my life and health insurance.
The biggest advantage of being able to live forever is being able to amass a fortune. I think I'd start out on life much the same way I have already. I'd go to school, become and engineer, work on advancing the state of the art, and make some money to save for retirement. Along the way I'd collect all sorts of things that could be sold hundreds of years later as museum pieces.
After a while of living frugally, saving, working, collecting and selling, I'd have a fortune that could sustain itself well beyond my desire to spend it. However, some diversity is in order. Since I can outlive governments and currency, that fortune needs to be tied up in valuable possessions in various countries. I would also make money by becoming a venture capitalist - funding promising research and startup companies that I found to be moving society in the right direction. The goal, always, would be to accumulate a massive fortune.
After 4-500 years, when I had enough money (and founding supporters), I would find a way to use that money to found my own country. From there, I would work endlessly to ensure that my nation secured the blessings of liberty, and see it prosper into a model for all humanity.
My nation would be funded with an endowment (which I and other founders would retain control of) that would be used to permanently run the government - no taxes necessary. The nation would be a constitutionally limited republic, with myself as a permanent chief of the court system, a protector of the limitations of the government. After the aforementioned 500 years, I think I could have developed a pretty solid bill of rights. Citizenship in my nation would have stringent requirements for knowledge and understanding of fundamental liberties.
From then on, that is what I would spend my time doing. Vigilantly protecting that nation from the natural urge to oppress the minority. It would take all of my time, and I would be happy to devote my eternal life to it.
It would be difficult to watch everyone I know and love grow old and die. I would forge new relationships only to see them grow old and die as well. I don't know how much of that I could take, and at some point I might have to keep everyone at arm's length. Not sure. That would definitely take some experimenting.