The ESRB requires all ESRB Website Council (EWC) members to add age gates to all websites for Mature-rated or Adults Only-rated games:
2.2 If the audio-visual content contains material from a Mature-rated or Adults Only-rated game (or one that is anticipated to receive either rating), display an ESRB-compliant age-gate prior to access or purchase.
2.3 Utilize an age-neutral approach (e.g., “Please enter your date of birth.”), when requesting the manual entry of a birth date from a visitor trying to access age-gated audio-visual content;
2.4 Employ reasonable measures (e.g., dropping a session cookie; returning visitor to the home page) to prevent visitors under the age of 17 (or 18) from re-entering a different birth date once they have initially been denied access to age-gated audio-visual content and utilize an age-neutral approach in the error message (e.g., “Sorry, you may not access this content.”);
Source: ESRB Website Council "Code of Conduct" (PDF)