A Fidelity Investments report from earlier this year indicates the total number of 401(k) millionaires reached 645,000, a 26% increase from the previous year. Millennials, aged 29 to 44, now hold an average of $82,600 in their 401(k)s and are saving an average of 14.4% of their pre-tax income annually. The strategy for wealth accumulation often involves consistent saving and investing in low-cost S&P 500 index funds, a method endorsed by Warren Buffett.