And more time for it to be mulled over. It was frustrating, but in college I found that the 16 week semester was rarely enough to actually learn something, only enough time to commit it to memory. It was often a year or more later that I'd have a sudden eureka moment while studying a separate topic. Perhaps something that was applying the unlearned material (like Physics I and Calculus, which were a semester apart for me since I took Chem I my first semester in college) or something related but not exactly the same (like Concrete Mathematics by Knuth et al. leading me to a realization about some aspects of calculus).

I had the experience of trying to relearn some math just a few years after first learning it in college and it was similarly much easier to learn it all a second time. I don't think I had a whole bunch of new experience. I think rather there must be some kind of residual understanding, even if it can't all be articulated cold.

Could this be a cognitive advantage of getting older?