As a 17 year old girl I would consider myself to not be scared easily in general. The other reviews were written when the first season was airing, and I would agree that the first season was just fine for pretty much anybody over the age of eight or nine.
My review is more focused on the second half of the second season. At that point the show gets very dark. The monsters (actually aliens) are real, and very evil. While the positive aspects still remain, (friendship, leadership, bravery etc) the villains become more evil and disturbing.
Characters die, in gruesome manners, and while not graphic it is clear what is happening. A main character sacrifices herself and is shot multiple times before dying. Other deaths occur, involving mass murder of a town. Again, I repeat these murders are not graphic, but very obvious.
As for other concerns such as sexual content or language, these issues are pretty minor and appropriate for the Y-7 rating the show received.
Again I stress that the second season of this show is very dark and disturbing, and in my opinion not appropriate for anyone under the age of ten or so. Even then I would watch the show before letting a child watching it, to make sure it wouldn't cause nightmares for that child. As a final note, the villains are never really destroyed, or unmasked in usual Scooby Doo fashion, adding to the creepiness especially for small children.
Finally I would conclude by saying that this show was enjoyable, and I watched every episode quite enthralled. I would compare it more to Buffy the Vampire Slayer or Doctor Who, and fans of such shows would probably like this show. Overall, teens and older tweens will probably enjoy.