I need help really bad I’m trying to write a story about immortals

So I need help really bad like anyone who is a immortal fanfiction writer I need your help because I’m trying to make the story really realistic and I am basically doing math with 12,000+ any age and it’s like really hard I’m trying to separate ages to 1990 to 1994. I’m basically asking can someone give me some tips on how to make the story really realistic
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