For practical reasons concerning app store rules, Tapas has it so that individual episodes are marked mature. If a whole story was set as mature, then it wouldn't be allowed to be shown on the app at all which would reduce visibility greatly.
If you want to denote that your series is for mature audiences, simply make a disclaimer as your first episode and include content warnings to let your potential readers know what they're getting into.
Minors will find a way to read nsfw stuff regardless. Even if there was an age verification system in place, they would find a way around that too if they really wanted. All you can really do is protect your bases with a disclaimer and state 'No minors allowed.'.