I think the parent poster meant "annual chance of dying after 80" as in, for any year 80+, you have a 50% chance of dying that year: it doesn't go up each year, so you don't "age" in that sense.
But this is not true. The life expectancy for an 80 year-old Caucasian American woman is over 9 years. The life expectancy a 90 year-old Caucasian American woman is less than 5 years.