My crack theory as to why the characters in Utena look like adults despite being teenagers is because that's how Utena sees them,she subconsciously believes herself and her peers to be adults,that's why Saionji and Touga especially look near indistinguishable from the male adult characters in the show.Perharps she sees them as looking as adults to soften the horrible things they and she are going through.Hell,maybe the reason they're all anime-styled is because Utena believes this story to be something akin to a show.