I think providing a simple "Yes, I am of age to read this in my country" in the user profile should be all the due diligence the archive requires. That flag would then open any fics rated over the appropriate level without continuous click-throughs. For users who don't want to have a profile (and there will be many - people can be very paranoid about such things), then they'd have to click the same age statement before they could open a "restricted" fic.

As a parent, I get very irritated by other people trying to restrict what my child can access. That's my job, thank you very much. This is something I could easily help her work around, should the need arise. It would, I'd think, also absolve the archive of liability.