WinterSnowfall: GOG being GOG. I mean guys, you have my credit card info... do you want a sample of my gray beard hairs too to know I'm over 17?
Well, you might be using your dad's credit card, and a fake beard...
I didn't see the warning on either Genesis Alpha One (which I actually just bought) nor the STALKER store pages, and I don't recall seeing the warning before either.
Anyway since you can lie in that warning page, I presume it is just for legal protection for GOG, in case you get severe trauma for the mature content in some game. Then GOG doesn't go to jail for it, but you do, for getting the trauma. You go to a juvenile prison for traumatized delinquents.