I have a B.A. in Applied Mathematics. Off the top of my head, I completed 7 quarters of Calculus(differential and integral calc, couple quarters of multivariable calc, theory of calc, advanced calc), a quarter of Ordinary Differential Equations, 2 quarters of Partial Differential Equations, a quarter of non-Euclidian Geometry, Complex Variables. I also had Abstract Algebra and Linear Algebra(proof course)....this was 30 years ago. Unfortunately, I have forgotten about 80% of what I learned.

When I tutored my nephew in differential calculus, I had to go online first to brush up, then teach him. This was quite depressing, considering that I aced this in high school without trying that hard.

My brain doesn't process info nearly as quickly as it did 30+ years ago. This is perhaps the worst part of again, IMHO. Thankfully, I am healthy. Things could be a lot worse.