Sure makes investing for the long term a little easier when your best competition is measuring success in decades, while you’re measuring it in centuries. “Uh, yeah, I bought IBM back in 1896…been accumulating it over the last 100+ years, the return’s been decent…” 
The concern is, that the patient might not have the option of refusing the experiment.
Suppose I were a methuselah, and you were not. If I said, “Yes, the fountain of youth exists, but you cannot have it”, would you take my word for it?
A lot of people would be tempted to concoct a conspiracy theory, and call their congressmen to demand an investigation. The methuselahs might not get due process.
Suppose methuselahs were discovered in Russia, but Vladimir Putin was not one of them. Would they continue to walk around freely? I am not sure.
I am not certain my own government could be trusted, and I am the most jingoistic person I know.
It’s a crime in Michigan. So, there are some states you’d want to avoid.
This is assuming you were able to develop enough cash to support yourself independently. Going from the various long-lived stories I’ve read, that tends to make for a fairly boring story, so most of the long-lived have been portrayed as people who still need to work, which means income taxes, drivers’ licenses, false identities… if you were the only one, it could be fairly easy over time, moving from city to city. If you were part of an organization, like in Methuselah’s Stepchildren, then you’ve got internal resources to maintain The Masquerade. The tricky part would be if in addition to being long-lived you were also unfortunately flamboyant and got your picture in the papers and stuff. Drawing attention to yourself would be a bad thing.
The problem isn’t with friends and neighbors. It’s with the official records. Official ID is required so much in every day life now that most people don’t consider it. Any major financial action requires a corresponding mountain of record keeping to prevent criminal activities like money laundering and terrorist funding. Just try walking into a WaMu branch office to open an account without having ID. Today you need a state issued ID and an SS#. Getting ID that will withstand an audit is becoming harder and harder. Eventually they’ll add fingerprints and photos to the required documentation. Next will be a DNA sample. No, this isn’t Conspiracy Theory 101. It’s all for our own protection.
I could probably never find it now because it was pre-internet, but I vaguely remember a thing from the 80’s or early 90’s that stated that if all forms of disease and physical aging were completely eliminated and the only cause of death was due to accidents, the most you could expect to live would be roughly 900 years before something got you, and the average life expectancy would be in the 600 year range.
Of course, I have only my faulty memory, roughly 20 years after the fact, to attest to this supposition.
So unless you’ve got some magical healing, ala Highlander or Wolverine, you’re NOT going to live forever, even in the perfect body.
And yet this seems to be a non-issue for any moderately competent criminal with some funds. No matter how much time, money and effort a government puts into this, it’s always possible to work around it. How hard is it to get State Issued ID and an SSN if you start 20 years beforehand?
Aside from such popular tricks as stealing the ID of a dead baby or whatever, a hypothetical methuselah would have all sorts of other tricks available to them, not least of which would be borrowing a baby, popping into half-a-dozen jurisdictions to register a ‘birth’ and then ripening these new identities over 20-30 years until they were ready for use. I’d be astonished if there were checks in place to catch this sort of long-term con - nearly all checks are instantaneous, along the lines of “Do you have a form B with details that match this Form A?”, working on the principle that hardly anyone would be able to fake both simultaneously.
I dunno, the major causes of death by accidents in this country is transportation, and if you’re never gonna die, why not just walk everywhere? Same thing goes for lots of dangerous stuff. To quote Cecil: