The drop is probably multiple factors, including aging. The study was done in 1975. Someone 28 years old would have been born in 1947. A 33 year-old would have been born in 1942 –the darkest days of WWII. Prenatal nutrition was likely an issue then. Ditto for those born earlier, as they had to deal with the great depression.

Also, the sample size in the first study was rather small, as they were just trying to answer the question, "Does the time it takes to mentally rotate an object depend on the angle of rotation?" (Answer: yes.)