According to the Mishna on our daf, if a man sends a messenger to deliver a geṭ the messenger is obligated to do so, even if the person sending the geṭ is elderly or ill. Simply put, we do not assume that someone who was alive in the recent past has passed away. This rule holds true not only for divorces, but also with regard to other laws, such a woman who is married to a kohen and continues to eat teruma – which is permissible only to a kohen and his immediate relatives – even if the kohen travels to far away lands.
Rava limits the rule presented by the Mishna to ordinary cases only – for instance, to an elderly person or to someone who has an illness from which most people recover, but not to someone who has reached 80 years old. Were the person who sent the messenger older than 80 or if he appeared to be on his death bed at the time, then we cannot assume that he remains alive for a significant period of time. Abaye counters Rava’s ruling by quoting a baraita which states that even someone who has reached 100 years old is assumed to be alive and his messenger would be obliged to hand the get to his wife as instructed.
The first response of the Gemara is that Rava has been proven wrong. Another suggestion raised by the Gemara is “keivan d’iflig, iflig” – once someone has lived beyond his expected years, he may very well continue to do so.
Estimating the life span of an individual about whom we have limited information is a question that is raised by both scientists and actuaries who work for insurance companies. Generally speaking we assume that people live an average lifespan, which is usually estimated to be 70-80 years. Tehillim 90:10 uses these dates, suggesting that the expected lifespan of a person has not changed much since biblical times; the higher average lifespan that we enjoy today is largely a result of the drop in death rates during infancy.
This assumption notwithstanding, once someone survives well beyond the average lifespan, he no longer fits into the normal statistical groupings, as it has become clear that he is in the category of the long-lived, about whom we cannot assume that death is imminent until we have definitive information about his death.