In what country is Miss used for under 18 and Ms. for over? In the US and UK, Miss just means "unmarried woman" no matter the age. So yes, its appropriate for an under 18y old, but its also appropriate for an over 18 year old.
"Cultural differences" is an easy scapegoat.
So if you’re presented with Ms. and Miss as options in a form, and neither has to do with marriage status, it’s natural that Miss will refer to the young.