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Can saving/investing 15% of your income starting age 25, likely make you a millionaire?
It depends on how much you save, how much your savings earns each year.  You can model it with a very simple spreadsheet:  Formula view:  You can change this simple model with any other assumptions you wish to make and model. This spreadsheet presumes that you only make $50,000/year, never get a raise, that your savings earns 6% per year and that the market never has a crash like 2008. The article never states the assumptions that the author has made, and therefore we can't honestly determine how truthful the author is.  I recommend the book Engineering Your Retirement as it has more detailed models and goes into more details about what you should expect. I wrote a slightly more detailed post that showed a spreadsheet that is basically what I use at home to track my retirement savings.