For those ASCIIng themselves why this works, 0x20 is 00100000 in binary, so it only flips one bit, increasing or decreasing the character’s value by 32. Capital ‘A’ is 65 and lowercase ‘a’ is 97, an offset of exactly 32! The rest of the uppercase and lowercase letters follow in order.
The correct response in this scenario is to challenge another family member (that you think you can beat) to an arm wrestle. If they lose, they are therefore also weaker than the 12 year old and can no longer talk shit.