How is age related change in IQ measured if tests use age specific norms? In a comment to this question about age related IQ decline in professors and scientists, the following comment was left:


  I noticed that lot of IQ tests (RPM or California) have different
  norms for age groups. So how do they measure decline of IQ. Is it decline
  of IQ as absolute mental strength? Or drop in numbers measured by
  specific test? – @ICanFeelIt 


I thought I'd phrase it as a specific question here:

If IQ involves age specific norms, how do researchers study age related changes in IQ?

See also this earlier question on age related change in IQ.
When studying adult IQ, general adult norms are often used. So for example, even if 70 year olds have lower IQ than 20 year olds on average, for research comparison purposes the same adult norms might be used to study age related cognitive differences.
Thus, raw-scores and IQ scores will be almost perfectly correlated (except for small adjustments to the distribution resulting form the conversion from raw scores to IQ scores). When studying change in intelligence across childhood, test raw scores can often be used. Of course, things get more challenging studying young children who are unable to complete tests suited to older children and adult.

Updated comments


Cohort and related effects: I agree with @what's point that assessing whether differences between age groups based on cross-sectional data is problematic. Such differences can be due to cohort effects, death, differential participation rates, and so on. It's much better where possible where possible to study changes in IQ using a longitudinal design where the same participants are studied over extended periods of time. If a cross-sectional design is all that is available then studying the effect of covariates, sampling participants in a thoughtful way, and reasoning about cohort effects are all important.
Scaling raw scores to IQ: @what suggests that you need to relate measurements to a standardisation sample in order to compare research with other studies. First, a distinction can be made between "intelligence" and IQ. IQ is standardised by age. Intelligence is not standardised by age. I am assuming that any studies interested in age related declines in IQ are implicitly actually asking about age related declines in intelligence. That is because by definition at the group level, IQ never changes with age, because age is by definition controlled for. Second, there is an issue of what is a useful metric for describing changes in  intelligence with age. I agree that IQ is a familiar metric with a clear mean and standard deviation. For that reason, it is often useful to use this metric with a single adult norm sample to study age related changes. That said, using raw scores would tell the same story regarding age related change; it's just that the metric would be a little less intuitive.