I remember when I was younger my mom was always against M games, but my dad and older brother didn't care. I usually played Halo with my older brother, sometimes alone. I just wasn't allowed to use a mic. Besides, when I was like 6, the majority of the rated M games were horror games...I didn't like to play them back then because they were confusing and gave me nightmares.
That's another thing, rated M games were more confusing for younger audiences than they were to older ones than they are now. Now, all you need to know is to run, jump, and shoot, then you're good to go.