It doesn't look like a user group promotion is possible as there's no age criteria. However, this is still possible if you don't mind adding people manually:
- Create a user group called Mature (or a name of your choice), but leave its permissions alone. You don't need additional permissions.
- Create a forum called Mature Content (or a name of your choice) and set it as private.
- In the node's permissions, explicitly set the View node permission to Allow for this new user group and leave everything else as is.
- Create an announcement thread asking those who are 18 or older and are interested in viewing mature content to post in the thread or send you a conversation confirming they're 18 years or older.
- Add the Mature user group as a secondary user group for those users.