For the first time, in 1912, psychologist William Stern used the formula "mental age to chronological age x 100" to show IQ. Let's say someone was 10 years old and thought like a 10-year-old, their IQ would be 100. But if, say, they were 13 years old instead of 10, their IQ would be 130. Check https://iqtestonline.io