I have played sci-fi games like that in the past at friends houses, and now at 13, I think that they are verrry different from Grand Theft Auto and stuff like that.
It depends on what's meant by sci-fi - zombies can be pretty freaky, nevertheless, always prove to die well...
But most of the Pegi 16+ ones seem to be ok.
However, I must warn you on the rating side of things - lots of DS games (e.g. Nintendogs, Super Mario etc) tend to be rated as 3+. I think that this is a pretty poor way of rating it, because it means that the game is
suitable for a 3+ year old. It doesn't mean that they can necessarily play them. Many of the overrated games
are overrated because of the content e.g. drugs, sexual features and violence.
Take 42 All Time Classics on the Nintendo DS - it's rated 12 because of the gambling... and even that doesn't encourage you to gamble - it's your standard game of cards!
Hope this helps a bit!