I mean, neither seemed young during the show but I was surprised that a storyline featured Flo retiring. Wasn't Selma older?

They both died of lung cancer. 3 years difference between them.

Understood but it doesn't answer the question

Selma Diamond was born in 1920, making her 64 when she died, and Florence Halop was born in 1923, making her 63 when she died.

Selma was older.

and Harry Anderson was 65 when he died he was born in 1952

They written Selma Diamond's death into the show. I don't remember if they did the same for Flo.