I suggest just reading MMM. It's all about savings rate. As an example, I live in Brooklyn, NY (one of the most expensive places to live in the U.S.) In 2016, I spent $32k ($24k was rent!) Assuming a $50k income, that'd be a ~36% savings rate, which requires a 23 year career.

Assume someone making $50k lives in not NYC, I imagine this scenario could be a lot better.

For a more extreme version of MMM, read http://earlyretirementextreme.com/. This guy (Jacob, who MMM mentioned in this interview) never earned much money (never over $50k prior to becoming financially independent), and retired after working 5 years. With a little creativity, most people can retire far sooner than standard retirement age.