You could for example add a MODAL (popup) that displays on initial page load, with information "Warning: This website has adult content", and buttons [proceed] and [exit]. Clicking "proceed" would simply close the modal popup, while clicking "exit" would navigate to another website, for example Disney.
There is no way to truly VERIFY that a user is 18+ of course. A 15-year old could still click to "proceed", just like on any other adult website. It's just a warning and precaution.
There is an example of "modal" in the demo link below (scroll down to modal):
https://demo.photo.gallery/examples/features/sample/
You can define a modal to open immediately on page load, as explain in the example.