I guess it depends on a number of factors whether you should let an underage person play this game.
For me I don't ever feel I am playing an M rated game.
My guess it was probably rated the same after Oblivion was re-rated.
But I am getting side-tracked:
1. If you object to violence and mild gore do not let your kid play it.
2. There are some disturbing fantasy concepts at play in Skyrim. If you feel your kid can handle this.. than fine let them play.. if not.. don't.
3. Kids under the age of twelve - fourteen. don't yet fully grasp morality, violence etc. and thus can react very unpredictably to what's happening on screen. Let me illustrate: I had a 9 year old nephew ( he is 22 now

) who was terified by "The Gummi-bears" but who could easily handle "Die Hard" (not my choice, blame his parrents). It didn't harm him in any way but a pre-teen may have different concepts about scares etc.
basically it's all about how a kid can handle these things normally. If you want to be really safe monitor during play, or do not allow to let them play at all. Like advlts every kid is different and thus will react differently.
One thing that must be emphasized kids are more aware about morality and violence than we give them credit for.. but might react differently depending on their personal character.
Or :
http://www.youtube.com/watch?v=e55P2XF38O0&feature=g-vrec&context=G237a88eRVAAAAAAAABA (turn down volume on the first seconds)
People don't want to be sword
