I am mature but my parents wont let me buy M games the only one I have pre ordered in Battlefield 3 so are M games suitable for 14 year olds

According to the ESRB, no. 17 is the age at which individuals may legally purchase M-rated titles for themselves. Otherwise, you would need to have your parents purchase the game for you, though such a choice is, at your age, at their discretion.
In many games, you have the option to disable mature or violent content like blood effects and explicit language.