A majority of software engineers don't make enough money to retire at 50. People who have retired so young tend to be very lucky in both employment and their investments. Most probably stayed unmarried, inherited significant amounts of money, and/or married into even more money. It also helps to be lucky enough to start with a $100k+ job at age 23 and never have any bad luck to set you back. I've met people who check some/all of these boxes, and even they seem to not be retiring at 50.