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.
Why would an immortal decide to take a pension?
Everybody wants money, that’s why they call it “money.”
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.