I've done grim and gritty, both in Warhammer and in D&D, and high magic grim and gritty is possible, I dispute that gritty is real world realistic, in my book gritty = dangerous, grim = dark, the good guys are loosing.
By default D&D have never been grim ro gritty but that is not to say that you cannot have a high magic or a low magic grim and gritty setting. I am sure that they are out there. I have never played in them, but Dark Sun sounds pretty grim and gritty from anything i have read about it and for that matter what about the Midnight setting.
That said, I have no longer any interest in such a setting, I am quite happy in the worlds of high fantasy, and I don't see anything unadult about it.