age markers

Sorry if this has been posted elsewhere - I did look but I couldn't find it. Is there a general guide somewhere for how to put a certificate rating on things we post to the site?

For example I am currently writing a multi-part essay on domestic abuse. The ones with explicit details I have marked 18 for obvious reasons but other milder chapters I have marked as 15 because I wasn't sure if the content would upset people, not because it's in any way violent.

Is there a list of general guidelines somewhere? I don't want to get it wrong.