Thomas Stanley and William Danko, authors of The Millionaire Next Door, found that only about one per cent of Americans become millionaires by the time they turn 35. This statistic is corroborated by another study conducted by Spectrem Group, which found that only four percent of Americans have a net worth of a million dollars or more. However, it's important to keep in mind that millionaire status isn't always synonymous with financial success. Many people who aren't technically millionaires still enjoy a comfortable lifestyle.