If I was a secret immortal how long could I keep the same US ID for before I get taken away for study. I'm thinking 150 years.
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.
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.
But if you're immortal, you can also think much longer term.
You can just keep working, or, if you're the truly immortal type, where you don't have biological needs, and are impervious to disease or injury, just sit back, since you don't need to pay for as much as a regular human.