They say that about leghorns, black sex links and California whites too. I have had all those and none died before 7 years old. All still laid eggs too.
After 2 years they slowed down and instead of 6-7 a week I got 3-4 a week but they still laid nice eggs.
While ISA are a production bird and...
No. There is something else going on.
I have 2 black sex links 5 years old still laying and eggs are fine. I have birds older than that (11 is my oldest) still laying and eggs are fine. In all my years keeping chickens age has never made the eggs taste bitter.
How is her behavior? Is she...
Free ranging they may be eating a lot more of something bitter like dandilions.
I know giving the birds certain things like flax seed is said to make the eggs taste odd.
I would take a walk around where they range and see what is out there in abundance.