I've never encounterd this, but have never been on an adult site. If it is being used for age verification, I would presume they would have to run it thru the credit bureau immediately, for date of birth. If there is any delay, like say come back later I would be wary.
If it is being used for age verification they should have terminal right there and can run name on card thru right then and there.
When asking for age verification on a porn site may be legit, but on any other site, some have you can be as young as 13 to purchase why would they ask that question, unless it was for I.d. theft. Hope you post where else you encountered that, because I never did , maybe I'm wrong and will learn something here, unless using moms, and credit bur. has an approval by me somehow on there.
Approval by me, usually means open no credit accounts unless contacting me first, but can be used, for a card w/a co user, to see what child is using for, or a commercial card for a company issued to a company user, i.e. and additional card.
Again would be wary if it were just to purchase a pair of shoes, or Nikes on line, then sounds like info gathering of some sort.