It's very hard to imagine you'll forget it, but it really is true. I used to be able to solve integrals while being too drunk to walk, but nowadays basic trig would require substantial effort for me. Time does that.

Agreed. I'm taking the online Stanford AI-class at the moment, and it's been half my lifetime since I used some of the necessary skills. Even just re-arranging a formula is slow going for me at the moment. But I am improving.