For adults (those above 18 years of age) there is a common test, the most popular of which is the WAIS (Wechsler Adult Intelligence Scale). The WAIS includes measures of verbal comprehension, perceptional reasoning, working memory, and processing speed. WAIS-IV has 10 subtests and 5 supplemental tests that summarize intelligence with two measures–a final IQ score and a General Ability Index. The WAIS is age-normed across age-groups up to and including 70–74 years. The WAIS manual enables a psychometrician to convert an absolute score on the WAIS to an age-related score to adjust for the gradual cognitive decline that occurs with age. A 70-year-old with an IQ of 103 is probably scoring on the WAIS test about the same as a 30-year-old of IQ 96. However, these adjustments for age become less reliable in the senior age groups because the variance in IQ’s becomes greater as we age, and applying the same correction factors for two different seniors between 70 and 74 years carries the implicit assumption that all seniors in that age range decline at the same rate.
The description of intelligence as the sum of two components: fluid and crystallized intelligence, helps to distinguish between neural efficiency (acuity) and accumulated knowledge (experience). However, the broad categories of fluid and crystallized intelligence contain many sub-competencies that may peak at different ages. To determine when each sub-competency peaks, Joshua Hartshorne and Laura Germine conducted a comprehensive study in which they tested 2450 participants between 16 and 89 years of age using the WAIS III IQ and Wechsler Memory Scale tests (WMS). (Hartshorne, Joshua, K. and Laura T. Germine. 2015. “When Does Cognitive Functioning Peak?” Psychological Science. Vol. 26 (4) pp. 433-443.)
The flow chart below displays the cognitive abilities sub-tests on the WAIS III IQ test that measure both performance IQ (essentially, the abilities that underpin fluid intelligence) and verbal IQ (the abilities that underpin crystallized intelligence).
The WAIS III Subtests Constituting the Full-Scale IQ Measurement
(Chart taken from my upcoming book Intelligence, IQ & Perception)
Reporting their results in 2015 the researchers stated.
Our results reveal considerable heterogeneity in when cognitive abilities peak: Some abilities peak and begin to decline around high school graduation; some abilities plateau in early adulthood, beginning to decline in subjects’ 30’s; and still others do not peak until subjects reach their 40’s or later.
From their study, the researchers found that the competencies that comprise fluid intelligence (lower left portion of the chart above) peak about two decades before the competencies that underpin crystallized intelligence (upper right portion of the chart). This is not surprising, because fluid intelligence is more closely related to the processing speed and power of the brain, while crystallized intelligence embodies knowledge that is accumulated over a long period of time.
The short answer to your question is that you can use the WAIS manual to compare age-normed IQ scores of people of different ages. However, as found in the research quoted above, the 70-year-old might have a higher absolute score on vocabulary and comprehension, while the 30-year-old might have a higher score on matrix reasoning and block design.
