Google has updated its SafeSearch, adult content filter, help documentation to say you should not show the age gate prompt to Googlebot. Google wrote, “For content behind a mandatory age gate, we recommend allowing Googlebot to crawl your content without triggering the age gate.”
Google wants to ensure Googlebot can see the content behind the age gate and Googlebot is not always able to complete the age gate. So if you have an age gate on your adult site, make sure you don’t show it to Googlebot.
Google said, “You can do this by verifying Googlebot requests and serving the content without age gate.”
Of course, you should make sure to clarify this content as adult, no matter what.
Forum discussion at Twitter.