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It depends on who draws them, I guess. There's quite a lot of underage content in permanent booru and even e621, even if absolutely realistically drawn.
Some will just draw it like this anyway. And if they are not allowed to draw underage dogs, then they'll draw "demons" the shape of an underage dog. Or dragons and state they're 2000 years old. ?

It's problematic.
There's probably no other possibility than to keep the uploaded and "written" parts of this forum clean as far as this works. And leave the thoughts to the individuals, those are at least not hurting anyone.
In show they are "teens". A 14 year old pony, is that not a grown up pony?
That's the logical / physical conclusion and absolutely acceptable, but a drawing doesn't .. hmm.. age. Or does it?
But in regards of MLP I see it the same as with Pokemon for example.
They're concept arts of something which can be "level 1" and "level 99". And evolve multiple times, look the same on level 1 and 99 tho.
As long as they don't explicitly use the "puppy" or "kitten" characters here, it's in my opinion acceptable. Same with MLP, if the ponies are not explicitly drawn underage -because- this is what the artist wants to visualize.