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