Since bitcoin is just a money why does bitcoin wallet sites has an age limit of 18 years old before creating one?? Does having a money has an age limit?? And since online sites can't even detect people who's 18 below , why do they even bothered it ??
websites do it because for certain contracts people need to be of a legal age to consent and agree to that services terms.
remember bitcoin is not a website. jst like a bank note is not a website. but as soon as you start using third party services that manage funds on your behalf. you have to give them consent and allow them to not be blamed due to outside events not in their control.
the reason why they even bother asking. is to protect themselves. and if it ends up the customer lied, then thats a breach of contract, which also protects the business