Common Sense Media says its not for kids. I played this and found that it's fine. There are some gory parts, in the first mission you discover one of the recon team from before, or what's left of him anyway. I played quite a bit and I will tell you now, you get very close with your team. I played it through once, well, I played about an hour or so, to get the fell of the game. Well let's just say that my whole team died in the second mission. I had to hire more and more soldiers. You really have to take good care of your troops. After I had learnt my lesson, I made sure I saved AFTER EVERY MISSION. I lost my best troop because I put him behind a tree. The first alien shot the tree down and then the 2 aliens after that killed him, at Major, Heavy. Luckily I had saved right before I did that. Some people may want to go "hardcore" and not save after they had completed a mission, or "ironman" or whatever it is, but I got really attached to my team an I didn't want them to die.
This is an amazing game. I played this for a couple of days and I loved every second. . . that my team was alive. Anyway, I think this is fine for mature 11 year olds and definitely ok for 12 -13 year olds.
P.S if you zoom up on some of the equipment in the research labs, it sometimes says some funny CAUTIONs.