Marking your channel as 'Mature' can be done during the creation of your channel, or edited within the personalize tab if you've already created the channel with mature content.
Initial Setup of Mature Content
In order to mark your channel as 'Mature', be sure to check "My channel contains (18+) Mature Content" before clicking 'Create Channel'.
For existing channels planning on releasing mature content, navigate to the personalize tab (the paintbrush icon on the left navigation in your content dashboard) and open the Channel Branding Settings Page. Scroll to the bottom of this page and check the box marked "My channel contains 18+ Mature Content"
For more information, be sure to read our content policy outlining Mature Content Guidelines and Enforcement.