I think the bottom line is that she doesn’t have an age, only a representation XD As with that famous pipe painting, a representation of an object is not the actual thing. Therefore… If you say she’s 18 here. She is. XD She’s whatever you say she is!
But it can be quite silly sometimes, hence the image.
Though a fun realization I had while making it, is that age spells are actually canon in MLP so it could actually be true - not that that was originally the intent, so is she actually 18 in the image?