Its not an argument, its true physiologically, or so its been reported. Certain types of reasoning peak (in terms of horse-power or raw CPU type performance) in your 20's, similar to raw athleticism. But there are a whole host of factors, around plasticity, memory management, and actual memory (ie, empirical data) that allow more real-world performance at a later age/stage. To continue the computer analogy, the fastest chip doesnt always win, if you have GPU, L1/L2 cach, improved bus-speed, SSD storage, and better written software....etc. Similary, in certain sports world-class athletes improve well beyond their peak physiology. Himilayan mountain climbing is the classic example of peak perfomance <not> being in your 20's.