Math seems to have a very ephemeral lifetime in the brain. I skipped a year of college once, and when I returned I realized I had to basically abandon any major with a math requirement, because I had seemingly forgotten everything.

I'm currently struggling with an online Machine Learning class (the Coursera one... at the tender age of 43), and I can only take it (so far, at least... just failed my first quiz... fortunately I can review and re-take) because I was rather obsessed with matrices, oh, about 28 years ago. "You mean I can rotate things in an n-dimensional space without using trig?"