There are two ways to add an age disclaimer to your site. You can either:
Use a lightbox
Create a landing page
Both options have a confirmation button and no other navigational options. That way, in order for visitors to view the rest of your site, they are required to click the confirmation button.
To create an age disclaimer using a lightbox:
Add a lightbox.
Click the Settings icon on the lightbox.
Select Yes under Automatically display lightbox on pages.
Click the Which pages is it on? field and select First page visitor sees.
Drag the Delay slider to zero seconds.
Disable the X icon toggle.
Enable the Close button toggle.
Click the "close button" on your lightbox and click Change Text.
Enter the confirmation text (e.g. Yes, I'm over 18).
Click the lightbox overlay and click Set Up Overlay.
Disable the Clicking closes lightbox toggle.
Customize your lightbox to create your age disclaimer.
Customize and setup your lightbox in the mobile Editor. By default, lightboxes won’t appear on your mobile site unless enabled in the Mobile Editor. This is because Google does not like intrusive pop-ups on mobile sites so having a Lightbox enabled may negatively impact your site’s SEO.
To create an age disclaimer landing page:
Create a landing page.
Click the Show More icon next to the landing page you created.
Click Page Info.
Click Set next to Make this your homepage?.
Add a text element.
Edit the text to reflect the notification. For example: "Are you over 18?"
Add a text button.
Click Change Text on the button.
Type the button text to reflect the user's confirmation. For example: "Yes".
Click the Link icon next to Where does it link to?.
Select Page / Anchor.
Select the page for users to be redirected to under Which page?.