Hm, maybe I'm missing something on how the tests interact, but if the older range up to 2008 on the SPM was falling, doesn't that tell you how the adults are going to turn out simply because they are closer to being adults than the younger counterparts?

A priori, it does seem like the older kid trend should be more relevant to adults than the younger kid trend.

However, British IQ gains might have a V-shaped relationship with age: solid gains in younger kids, lesser gains (or indeed losses) in teenagers, and a return to higher gains in adulthood. As written that probably sounds like a wilful disregard of Occam's razor, but there is some precedent.

I went back to Flynn's original 1987 article "Massive IQ Gains in 14 Nations" and looked up Great Britain. The only Matrices results seem to be for the SPM, but as well as the familiar 1938-1979 results, there's an adult IQ gain estimate. Flynn got it by comparing a 1940 sample of militiamen (average age 22) at a WW2 training depot to the 15½-year-olds in the 1979 sample, adjusting the gain estimate to partially offset the teenagers' age disadvantage.

Comparing the adult(ish) gains to the child gains reveals something like a V-shaped trend: ages 8-11 gained 0.25 points/year, ages 12-14 gained 0.11 points/year, and the 15½-year-olds outpaced the militiamen by 0.18 points/year. In this case, the older kids' gain rate was no better as an estimate of the (pseudo-)adult rate than the younger kids' rate.