There's no question of the chart being "reliable"; it's not a prediction, it's a definition. It's just illustrating the principle of IQ, which is IQ = mental age / chronological age * 100. If you're 4 years old and function as an average 6-year-old, you have an IQ of 150. Therefore, since IQ doesn't change, you're going to have a mental age of 9 when you reach the age of 6. Because this definition becomes useless once you reach "mental adulthood", IQ tests on adults are always calculated based on a chronological age of 16. Yep, it's the only statistic in the world that's made for kids! I always thought that was kind of cool.
