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 ??
What wallet did you trying to create? Afaik blockchain web wallet did not applying age limit to their users so you can create it one without worry how old are you.
You just simply to choose the wallet service that didn't set an age limit , however it's just the matter of policies.