How does raw IQ change over the lifespan at the group-level?

Question

In the classic review of intelligence research (Intelligence: Knowns and Unknowns), Neisser et al (1996) write the following

Stability. Intelligence test scores are fairly stable during development. When Jones and Bayley (1941) tested a sample of children annually throughout childhood and adolescence, for example, scores obtained at age 18 were correlated r = .77 with scores that had been obtained at age 6 and r = .89 with scores from age 12. When scores were averaged across several successive tests to remove short-term fluctuations, the correlations were even higher. The mean for ages 17 and 18 was correlated r = .86 with the mean for ages 5, 6, and 7, and r = .96 with the mean for ages 11, 12, and 13. (For comparable findings in a more recent study, see Moffitt, Caspi, Harkness, & Silva, 1993.) Nevertheless, IQ scores do change over time. In the same study (Jones & Bayley, 1941), the average change between age 12 and age 17 was 7.1 IQ points; some individuals changed as much as 18 points.

Typically, IQ scores are standardised for age and thus remove age related differences.

My question is

What is the relationship between age and raw intelligence test score at the group-level from around age 5 and into adultyhood?

From what I've read, I understand that at the group-level intelligence increases monotonically with age during childhood and that it levels-off at some point (perhaps around 18?). Also, below a certain age (e.g., in infancy), completely different tests are required to measured intelligence. I also understand that individuals may experience various trajectories (e.g., some might plateau earlier, etc.), but I'm interested in the group-level. Thus, an ideal answer would probably provide a sense of the functional form of the increase in intelligence over childhood and adolescence.

My first thought for an answer was to look at the test manual for a standardised child intelligence test, such as the WISC, but I don't currently have easy access to that.

References

• Jones, H. E., and Bayley, N. (1941). The Berkeley Growth Study. Child Development, 12, 167-173.
• Moffitt, T. E., Gabrielli, W. F., Mednick, S. A., & Schulsinger, E (1981). Socioeconomic status, IQ, and delinquency. Journal of Abnormal Psychology, 90, 152-156.
• Neisser, U., Boodoo, G., Bouchard, T. J., Boykin, A. W., Brody, N., Ceci, S. J., Halpern, D. F., Loehlin, J. C., Perloff, R., Sternberg, R. J., and Urbina, S. (1996). Intelligence: Knowns and unknowns. American Psychologist, 51:77-101. FREE PDF

Answer 1

In general, the introduction section of McArdle et al (2002) provides a good literature review on the topic (see link to PDF in references).

Early research by Jones and Conrad (1933)

McArdle et al (2002) extract a quote from Jones and Conrad's (1933) summary of the literature based on large cross-sectional studies and also the classic Army alpha tests:

"The chief characteristic of the curve may be summarized as involving a linear growth to about 16 years, and a negative acceleration beyond 16 to a peak between the ages of 18 and 21. A decline follows, which is much more gradual than the curve of growth, but which by the age of 55 involves a recession to the 14-year level. (p. 239)"

Cattell on Crystalised and Fluid Intelligence

Also, see Figure 1 in McArdle et al (2002) which extracts a diagram from Cattell (1987) that shows Cattell's theorised relationship between age and two types of intelligence, crystalised and fluid. The theory suggests that fluid intelligence increases at a faster rate and peaks at around 18 and then shows a steady decline, whereas crystalised intelligence increases throughout the life span, with the rate of learning decreasing over time, and arguably approaching a plateau around 50 before commencing a very gradual decline around 60.

References

• Cattell, R. B. (1987). Intelligence: Its structure, growth and action. Amsterdam: North-Holland.
• Jones, H. E., & Conrad, H. S. (1933). The growth and decline of intelli- gence: A study of a homogeneous group between the ages of ten and sixty. Genetic Psychology Monographs, 13(1), 223–275
• McArdle, J., Ferrer-Caja, E., Hamagami, F., and Woodcock, R. (2002). Comparative longitudinal structural analyses of the growth and decline of multiple intellectual abilities over the life span. Developmental Psychology, 38(1):115. FREE PDF

NOTE: After posting the question, I did a little research and found the above research. I've self-awarded the answer for sake of closure, but I'm happy to award to someone else with a better answer.