Require webcam, have the visitor capture and upload imagery of their genitals (it must be from the attached image-capturing device), have site app determine whether genital (or breast) size is large enough to be considered an adult.
Duh.
"If you are Asian, click here for slightly less stringent size verification" lol
Seriously, there are cigarette vending machines in Japan that use cameras to try to determine age based on facial features, height and so forth. Even those have trouble with accuracy.