While piaget proposed that formal operational thought is not fully settled in until around 15 years of age, evidence suggests that some people never fully employ formal operational thinking. studies show that only of college students and adults achieve formal operational thinking, and other estimates run as low as forums?

The answer to this question is  40-60% ; 25%
Other estimation is based not based on the raw intellegence possesed but rather based on whether they could use the intelligence for real life purpose. Studies shows that even though young adults already fully developed the capability for formal operational thinking , onle a quarter of them actually got used to doing it.