Nothing magical, because when people age they become less healthy and able to work/contribute - it is only ethical to offer them the last ~10-20 years of their lives as reward for their hard work. The average age of death in the US is 78

And who pays for the benefits the elderly (or as a matter of fact, the whole society) enjoys? Amenities, welfare, health insurance, etc. - taxpayers, exactly the demographic (before 65 - healthy & able-bodied) FIRE is encouraging to stop working, thus avoid paying income taxes.