We've made a slight edit to rule 2c in our rules page. The intent remains the same but the wording is hopefully more clear.
The previous version was:
"Do not upload suggestive, questionable or explicit content of characters generally considered to be underage."
The new version is:
"Do not upload suggestive, questionable or explicit content of characters portrayed as, or intended to be, underage."
It was brought to our attention that the old version was not clear enough and left room for misunderstandings. Let us be clear that the our intent was always to allow not-safe content of characters that are usually minors as long as they are aged up in a convincing manner.
As for what counts as "convincing", there's a few factors. For humans and anthros it's usually easy and relies mostly on proportions. When it comes to ponies or some races in the show, it gets more difficult. Proportions or size can sometimes not be enough, depending on the artist's style and even within the show's style itself, so we have to take into account things like context, maybe anatomy and finally but most importantly, artist intent. Manebooru considers it very important to respect artists' wishes and preference in regards to their own art and we're gonna keep trying to uphold that as much as possible.
There are certainly edge cases that can be seen one way or the other and the team acknowledges them. We try to draw a coherent line but it's not going to be perfect. For anyone that finds these cases upsetting you are free to report them or approach us and we can at least give you our reasoning, your might disagree anyway but try to acknowledge where we're coming from and leave it at that.
For anyone who in no way, shape or form wants to see this content that classifies as aged up, or doesn't want to risk the edge cases, we recommend hiding tags like "older apple bloom" or "older spike". It is our policy that any not-safe content involving certain characters must, by necessity, have these tags in them, and we enforce it very strongly, so it should be pretty safe.
The topic is up for discussion for anyone that wishes to do so. If you want to link to examples or something, please make sure to spoil it properly.