If you allow followers its obvious that you are 18+.
To allow followers on facebook Go To https://www.facebook.com/settings?tab=followers
Or Alternatively You Can Go To Account
Then Followers Tab.
Now Here you have to change the settings from “Friends”
for allowing friends of friends and strangers to follow
Also you have to allow public to like your posts and they can also post their comments below your each post.
After Setting subscribers feature on, you have to do one more setting with your facebook profile which is " Make your posts publicly visible