It’s not like there are people checking for immortals, I think it would be flagged by a dmv employee or something when they dont believe a clear 21 year old is actually 150. Let’s assume it’s current day im caught and not bring speculation on what the US is like in the year 2139 is like.
You won’t be taken away for “study”, you’ll be taken away for pension fraud. Probably much earlier than 150.
Why would participating in studies be bad, though? Major pharmaceutical companies would pay you an absolute fortune in exchange for participation and you could advance medical science tremendously. You’d be a hero and get incredibly rich in the process.
If you’re immortal in the sense that you don’t age it would be dangerous to be outted. 8 billion jealous mortals would be an issue for you.
I think you’re overestimating how generous a government or research group would be if they found someone they truly believed to be immortal.
Why would an immortal decide to take a pension?
Everybody wants money, that’s why they call it “money.”