Does IQ peak in the mid-to-late 20s and then decline and how does this look for "crystallized" vs. "fluid" intelligence?

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It seems that fluid intelligence rises with age during people's twenties, then holds roughly constant  in one's thirties and forties and starts to drop after 50. On the other hand, crystallized intelligence does not seem to drop with age, but it also shows the highest correlation with age during the twenties. The way we tested this was by dividing our sample into three categories: younger (age < 30), middle aged (age >= 30 & <=50) and older (age > 50). We then calculated scores for fluid and crystallized intelligence for each of our participants and correlated those scores with age. We show these correlations in the table below. In addition, we have plotted a smoothed line of best fit depicting the relationship between IQ and age throughout life span (from twenties to seventies, as this was the age range of our participants). 

		Correlation (r)
Correlation between fluid intelligence and age	Younger (<30)	0.14
Middle aged (30 to 50)	0.01
Older (>50)	-0.22
Correlation between crystallized intelligence and age	Younger (<30)	0.24
Middle aged (30 to 50)	0.06
Older (>50)	0.01

This is how it looks graphically:

What do the other studies say?

Research on fluid and crystallized intelligence reveals distinct age-related patterns. Fluid intelligence seems to develop earlier, peak in early adulthood, and then decline thereafter, with accelerated decline after age 55 (Kaufman & Horn, 1996; Li et al., 2004). Crystallized intelligence increases through the 20s, plateaus until around age 60, and then declines (Kaufman & Horn, 1996; Horn, 1980). Our results are quite similar to these general trends found in the literature.

Takeaways

• Fluid intelligence seems to be rising throughout younger age, then remains stable in the adulthood and steadily declines in older age (after 50)

• Crystallized intelligence also rises during early adulthood, but then plateaus. 

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