I guess I learned it subconsciously.
...Like all actual rules of grammar.
Nah, plenty of grammar gets taught in elementary school.
Yes, but only rules which either (a) have already been learned subconsciously by the students, and what they're actually being taught is just to describe, understand, and be aware of the rules, rather than to apply them (which they already knew); or (b) aren't actually rules of grammar, but rather just rules of formal style.
Some rules, especially the esoteric and specific ones, are certainly learned consciously. Where to put modifiers like "only" in a sentence, for example. Or the difference between "who" and "whom", and "me" and "I".
The rules about where to put
only are really, really not grammatical rules. They're advice about clarity.
The rules about
whom have the structure of grammatical rules, anyway, but they're basically not meaningfully part of the grammar of anyone's English anymore. They're highly-artificial rules for particular formal genres of writing. The rules about
I and
me are also grammatical rules in structure, anyway—although they mainly boil down to 'don't use a certain subset of constructions in formal style', since the actual behavior of
I and
me in the grammar is much more complicated than that.
A rule like adjective order therefore seems quite remarkable: it's all learned subconsciously and I never hear anyone screw it up.
That's really true of most grammatical rules. The reason it's so remarkable, I think, is because it's so non-obvious that there even
is a rule. I mean, it's pretty obvious that there's a rule for, like, where you put the object of a verb, and how to form a yes/no question, and so on. But the language could get by just fine without having any rules about adjective order, and it takes some effort to notice that there even is a rule.