I think it's based on your birthday. If you have not set a birth date (or set one that makes you < 18 in $CURRENT_YEAR) then you will not see NSFW content.
I'm not sure if it defaults to opting into NSFW content based on your birthday. That seems like a poor choice of behavior. It's possible this only recently became an issue with the introduction of the "for you" tab recommending content with little to no filter.
I'm not sure if it defaults to opting into NSFW content based on your birthday. That seems like a poor choice of behavior. It's possible this only recently became an issue with the introduction of the "for you" tab recommending content with little to no filter.