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.
I think you’re overestimating how generous a government or research group would be if they found someone they truly believed to be immortal.
Although getting to that point would be quite difficult. Someone being truly immortal is so far out there and so difficult to prove that it’s much more likely to be something more mundane. Like identity theft, or a clerical error.