This is a hard line rather than an age-rating question — content whose primary purpose is titillation, or that meets a common-sense bar for pornography, is rejected outright regardless of any age gate applied to it.
Dating, art, or "adult humor" apps that include explicit user-uploaded images without adequate moderation are the most common cause.
"Your app contains pornographic material, defined by Apple as content whose primary purpose is sexual arousal, which is not permitted on the App Store per guideline 1.1.4."