When I was 12, I'd been BASIC programming for two years, and of course I used variables constantly (variably?) yet I didn't understand this simple crucial fact

When I was 12, I'd been BASIC programming for two years, and of course I used variables constantly (variably?) yet I didn't understand this simple crucial fact about algebra, which is that the letters are variables. I had a bright yellow pocket book titled "Calculus" and I used to carry it around in the hope that people would notice, but I couldn't understand page 1. I was into substitution cyphers and I devised a few, and I read about numerology, and at one point came up with the theory that the letters in algebra stand for numbers based on their position in the alphabet. Despite being steeped in all these instances of symbols standing for other things - and maybe even using x and y as variables in my programs - I just didn't imagine that algebra might be doing the same thing. It had a sort of stony, inscrutable face, permanently mysterious.