It will probably be 3 or 7. TP only got 12 because of the more darker, "scary" themes.
I'm assuming you're talking about PEGI ratings, since these don't exist for ESRB.
I expect to see at least E10+ if only because ESRB ratings feel like they're getting stricter and stricter. The act of killing enemies with a sword, even if it isn't particularly graphic, may earn it that rating by itself.
It could also easily end up with a T. After all, that's what Smash Brothers got.
M is just completely out there. Nintendo doesn't make M-rated games, as they try to reach broad audiences with their work.
I'm thinking E10+ because the "action/fighting" parts of Skyward Sword look like the fighting in Wind Waker, even though Link is older, and has Motion Plus. Maybe Link being older in this game (Miyamoto says this is the oldest Link so far) will cause this game to get a T-rating.
I don't think that's how that works.
If anything, Link being younger would get it a higher rating, because violence toward children is more risque. But I doubt it matters overall.