When your id says you're 100 and you look 21 it's going to cause issues.

You want to get away from ever needing an ID. The wealth you gain from compounding interest should allow you to hire accounting experts who will handle your transactions and hide your wealth among shell companies. I think once or twice you could go with the "this is my child, me Jr" routine, but eventually you need to have some kind of emissary who conducts business on your behalf while you cycle through fraudulent ids and move around every 20-30 years.

If your goal is to avoid that, and you look 21 permanently a la Highlander, you probably want to get new one every thirty years or so, starting over as a "runaway teen" or "refugee" who lacks identity documents with a nominal age of fifteen.

Or just commit identity theft. That one you could probably do once a decade, or more; just keep a running file of unsolved disappearances of children and teens and pull another one out whenever the age more-or-less fits.

"Oh, you must be thinking of my grandfather, we have the same birthday"

I was watching this real-life documentary called Highlander about this dude Connor McCloud of the Clan McCloud. He is immortal, but he has to sword-fight people because if he gets his head chopped off, he isn't immortal anymore. Anywho dude changes names every time someone gets too close. There was also a TV documentary by the same name about his cousin Duncan. Duncan is a bit more loose with it but they pack up and move around a lot. You should check it out, not Highlander 2, though; you can skip that one.

when they dont believe a clear 21 year old is actually 150.

This happens much sooner. You have any ticket, anywhere (bus, flight, stadium, speeding...) and sometimes they would check your face with your written age for plausibility.

The oldest person on record died at 122, and there's reason to think that there was fraud involved and she wasn't actually that old. By the time you were in your hundred-and-teens, you would have attention from scientists even if you looked your age. They wouldn't be forcing you to undergo medical testing if you didn't want to, but I think they would resort to force sometime in your hundred-and-twenties. If you didn't look your age, you'd have attention much sooner than that but people would think you stole someone's identity (that's what they think the 122-year-old person might have done) and not that you were immortal.

Btw there's a movie about that: The Age of Adaline.

There's a movie about this:

The Man From Earth

The circumstances where you’d be most likely to run into issues is where age plays an active role—e.g., Social Security or insurance. But those are probably avoidable if you’re careful. Otherwise, there’s no law against being really old or looking young, if you’re not trying to claim age benefits—for anyone else where the date of birth wasn’t relevant to their job, they’d probably just ignore it or assume it was a typo.

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.

Thats an interesting one if u happen to have happened upon a path to immortality lmk with that power i can help you rule the world. Forever.

It would be very easy. There are many places where money is all you need. Living in a shithole like the USA is the last place you want to be. Go anywhere you find Russian oligarchs or their kids. There are many micro nations that would gladly let anyone print any name they would like for a fee.

You're looking at frequent IRS Audits and verification requests from Social Security and what not, but TBH if those people keep their mouths shut and you don't tell anybody you'll probably never get taken away in most developed nations.

The thing about it is you'd have a difficult time convincing anybody you're really that age, and the companies that are capable of studying you aren't actually competent.

You're more likely to just get deported somewhere at random.

Why keep the same identity instead of assuming a new one every60 years?

People always say they would get a new identity but never how