Fluid intelligence, working memory, and processing speed all peak around age 23, while crystallized intelligence (closely associated with verbal intelligence) peaks in the 20s and holds or increases somewhat across the lifespan, absent other pathology. Our capacity to use crystallized and fluid intelligence effectively increases over the lifespan (this is the Wisdom leg of Sternberg's Triarchic Theory of Intelligence, also known as Successful Intelligence).
Your source didn't really note anything about the IQs of the test subjects, though it did say something about W-scores that were +/- 1SD. Were the subjects in the study representative of the population? If so, this means that most of them had IQs between 70 and 130, with most between 85 and 115.
I'm going to speculate that working memory, processing speed, etc., may peak sooner in people with lower IQs and later in people with higher IQs. I have little evidence for this statement, and will look it up later when I'm not working. If someone knows of a study on this exact subject (or a review, ideally), I'd be grateful if you could post a link here.
Either way, I don't think one can generalize cognitive results from people in the middle of the distribution to people who are >3 SD from the mean.
