This sentence means:
“Most older people do not live for very many years after reaching age 80. Here are four reasons why.”
Breakdown
“Most older adults” = most elderly or older people.
“don’t live much past 80” = they often do not live many years beyond age 80.
“past 80” means after the age of 80.
“much past” emphasizes that the age is not far beyond 80.
“Here are 4 reasons” = the writer is about to explain four possible reasons for this.
Tone
This is written like a headline or social-media title. It is designed to make people curious about the four reasons.
For example:
Most older adults don’t live much past 80: Here are 4 reasons.
A simpler version would be:
“Many people don’t live very long after turning 80. Here are four reasons why.”
One important point: as a factual claim, “most older adults don’t live much past 80” depends on the population and statistics being discussed, so the headline by itself shouldn’t be taken as universally true.
