I went my whole life learning to use "less".
Not just signs in stores, but math and general computer use. "Less than & greater than", for example. I have been into programming since the 1990s, so less than / greater than was a common calculation to perform.
I was in my 30s when someone told me about the word fewer.
In programming, I'm sure that if you knew the exact amount of something was fewer than another amount, you still use less than to check.
I know to use the word "fewer" in speech.