Though I do agree kids under 18 shouldnt play mass effect, that is because of the sexual scenes in the game, there is stripper aliens dancing, and a alien which comes and starts "touching" the main character in sexual ways. Though you dont see anything more than just touching for afew seconds, you know from their conversation and actions, what they get into when the screen fades out black.
I think the game being rated M if fine, parents just need to take action and find out what M stands for, I mean, I played GTA3 when I was 13-14, but that was because the game was super violent (and some language) but I didnt play san andreas when it came out, there was WAY too much language, way more violence, and actual built in sex games, that game needed to be rated A. San andreas was worse than mass effect.