Stupidity (can we still say that?) is really a decline or lack of fluid intelligence, the horsepower that lets you think abstractly and creatively solve new problems. Most older people have learned stuff over the decades and score rather better at crystallised intelligence (see the Wikipedia article for more on fluid and crystallized intelligence).
What does the world of science have to say? It's not completely easy to find out, but after some digging I found the diagram below (from here) - where the T-scores on the vertical axis are rescaled IQ scores, as shown further down this post. It shows the sad story of someone who had an IQ of 120 at their mid-twenties peak (T-score 63) but was merely average at 60.
It seems we lose about 0.6 IQ points per year from a high point when we're 26, a figure consistent with this Aberdeen/NHS study. How depressing!
Whatever my IQ was at age 26, it's now 23 points lower. And yes, I had to resort to pen and paper to work that out ...
|Age decline in fluid intelligence - around 0.6 IQ points per year|
|Translating between the different statistics (IQ, Z, T)|
You might want to take a look at this LessWrong article which has a different graph, but one which is depressingly and scarily consistent.
* Understanding the Stats (Descriptive Statistics and Psychological Testing - Stephen E. Brock, Ph.D., NCSP)
How is IQ scored?
IQ scores are a standard score with a mean of 100 and a standard deviation of 15.
Z-scores have a mean of 0 and a standard deviation of 1.
Z-scores can be transformed into IQ scores by multiplying a given Z-score by 15 (the standard deviation of IQ scores), and then adding 100 (the mean IQ score). For example, a Z-score of –1 equals an IQ of 85 [100 + 15(-1) = 85].
Transforming a Z-score into an IQ score: IQ = 100 + 15Z.
What are T-scores?
T-scores are standard scores with a mean of 50 and a standard deviation of 10.
Z-scores can be transformed into T-scores by multiplying the Z-score by 10 (the standard deviation of T-scores), and adding 50 (the mean of T-scores).
For example, a Z-score of –1 equals a T-score of 40 [50 + 10(-1) = 40] ,,, and an IQ of 85.
Transforming a Z-score into a T-score: T = 50 + 10Z, and IQ = 1.5T + 25.