Well, this was my 12-year olds first m-rated game. I watched him play for an hour or so, praying that he wouldn't see something gruesome and have nightmares. Buy in all honesty, Halo: Reach is a relatively mild game, and is gun as well.
First you need to know that we live in a Christian home, and that out family loves playing video games together! I grew up playing the old Super Mario Bros on my NES. My wife and I don't generally let our 12-year old play m-rated titles without our supervision, but this was not a problem. The blood was so unrealistic, I didn't realize it was blood until my son pointed it out to me. Mild, PG-rated language. Some violence, but if you let your kids watch Star Wars, there's nothing new here. No sex, I don't recal any alcohol.
The only real problem, and probably the reason for the N rating, was the online interaction. Generally there was some mild cussing, no worse than the actual game. But every once in a while there would be some crazy little kid cussing like a sailor, trying to act all grown up. It was kinda funny, but I didn't want my kid hearing that. Fortunately, you can mute players online, and most of the gaming community is friendly and do not cuss.
So, Halo: Reach is a great game in general, and I would have no problem letting a 10-year old play this. Just be prepared to mute some players, and it you have time, play with your kid. Bond with them.
One more thing, I like the role models and how they are willing to sacrifice themselves for the greater good. When my son saw this, he noted that this was courageous of them and that they were acting heroically, and how they almost represented Jesus. Of course, Jesus never defended a planet from aliens, but the morals are there just the same.
So, if you want to treat your kid, I recommend this game whole heartedly. I'm not a big shooter fan, but Halo is a blast! This is a great "first m-rated game", and was even milder than Assassin's Creed.
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