That’s stupid, though. If you can explain 11% of the variance of some noisy phenomenon like cognitive and behavioral flexibility, that’s noteworthy. They tested both linear and quadratic terms, and the quadratic one worked better in terms of prediction, and is also an expression of a meaningful theoretical model, rather than just throwing higher polynomials at it for the fun of it. Quadratic here also would coincide with some homogenizing mechanism at the two ends of the age distribution.