You make fun about this law, but did u ever think about it?
There is a very good reason: Human brain needs time to the early twenties to be fully grown. This mean alcohol and cigarettes which are affecting harder still growing brains shouldnt be consumed by people under 20 - 21.
Doing porn is something which has to do with self-determination and for that mostly humans over 15 around are able to decide - 18 is just a safe option cause everyone is different.
Driving a car or doing porn is totally different from substances your body has to deal with.
