I think the game is quite good, harking on many of the fun and rewarding game elements that I loved in the Diablo games back in the day, or that many people find in the World of Warcraft game, only with less online exposure.
The game does have a bit of language, but nothing shocking or outrageous that a kid in their teens wouldn't face exposure to anywhere else. A curse here or there, nothing more.
The violence, with decapitations, and body parts, and burning death screams, does sound quite gruesome on paper, but in practice, the caricature-ish manner in which the whole game, including the violence, is handled definitely reduces the impact. Even the human characters in the game are non-realistic enough to easily separate any tie to reality. I find Call of Duty to be much more suggestive in terms of violence.
I wouldn't give this game to a kid under 10 or maybe 12, but any child in their teens, with enough supervision, should find a lot of safe enjoyment with Borderlands.