Didn't clear the right cookie. Your attempt to clear all cookies didn't actually clear all the cookies. You still have a stale cookie telling the forum you are under age.
The cookie that needs to be cleared is in the domain cdprojektred.com and its name is "cdp_agegate_witcher".
In Chrome, go to Settings / Show Advanced Settings... / Privacy / Content Settings... / All cookies and site data
In the Search box, enter cdprojektred; you'll get two entries, for forums.cdprojektred.com and cdprojektred.com.
Select cdprojektred.com, find the button for "cdp_agegate_witcher", click on it, and click "Remove".
Go back to the forum, try to enter, and get prompted by the age gate again.