How to add the age disclaimer (such as 18+) to the website?

To show such a disclaimer, use the T658 block from the "Other" category. In the Content tab of the block, you can specify the disclaimer text, texts for buttons to agree and decline, text for deny message.

The user will not be allowed to view your website until they click the age confirmation button.

To include the disclaimer in all website's pages at once, add the block to the header or footer of the website.