How does raw IQ change over the lifespan at the group-level? 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, &amp;
  Silva, 1993.) Nevertheless, IQ scores do change over time. In the same
  study (Jones &amp; 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., &amp; 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
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., &amp; 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.