Yea you can download it. I had an account that wasn't 18 a while back and I couldn't download M rated demos and shit but as long as I PAID for the M rated content, it let me download it. Since you can already download M rated stuff, you should be safe. Just to be sure, you can go to your account settings and promote a child account. I did cause I wasn't sure how long I had owned the Xbox so I checked and I was able to.