9. The researchers also included a vocabulary test, which serves as a measure of what is known as crystallized intelligence — the accumulation of facts and knowledge. These results confirmed that crystallized intelligence peaks later in life, as previously believed, but the researchers also found something unexpected: While data from the Wechsler IQ tests suggested that vocabulary peaks in the late 40s, the new data showed a later peak, in the late 60s or early 70s.
The researchers believe this may be a result of better education, more people having jobs that require a lot of reading, and more opportunities for intellectual stimulation for older people.