Putting an M rating on the game means that the parents will need to buy the game, or the kids will need to borrow the credit card so they can buy it online.
There will be kids playing and at times it will be bad for "us" and other times it will be bad for them.
I am not concerned in any way about the level of violence and gore in ESO. I was also not concerned about it in Skyrim, Oblivion, or Morrowind. My 10 year old son and 12 year old daughter play Skyrim (the boy much more than the girl). They play in the living room and it is something we do together (look out!, what's that over there?). It has been a great experience for us as a family because we do it together and talk about it and all that.
What would concern me in ESO, or any other MMO, is the people that they would be interacting with. I'm not talking about getting lured away by a sixual predator (although it certainly will be a factor for some), I am talking about just the day to day horribleness that will be scrolling through the chat window. Racism, homophobia, sixism, and everything in between. There are people that will say horrible things and not care even if they are aware of the fact that they are speaking to children.
Having a real age gate that actually prevents children from playing will not stop the putrid hatespeak that will flow through your headset on a console. It only means that the voices will be deeper. Or that the fingers will be fatter and hairier on PC.
I've played with a lot of great people young and old, the only difference was that some had to go to bed because it was a school night, and others had to go to bed because they had to go to work in the morning.