There were a lot of changes from the book to the movie, but overall it was a decent adaptation. I think it is appropriate for anyone 13 and up, but a mature 12 year-old could handle it just fine as well.
This is a science fiction dystopian action film, so it's not surprising that there is some violence involved. The violence is perfectly PG-13, although it can be a bit intense at times. There are many shootings, killings, fights. There are dead bodies shown multiple times, although they aren't particularly gory. The teenage characters are forced to fight each other multiple times, and they are told to show no mercy and fight until the opponent cannot continue. The main character's parents sacrifice their lives in an attempt to save her. The movie focuses on one faction in particular, Dauntless. Dauntless is known for bravery, so the characters have to do particularly daring tasks, such as jumping into a chasm without knowing there is a net below, or playing a game of Capture the Flag with weapons that, while not real guns, feel like real gunshots when shot. In one scene, a teenage girl is forced to hold on to a railing for a specific amount of time or else she falls to her death in the chasm below. In another, Tris is captured by three males in masks who attempt to throw her off the cliff. One male character commits suicide (offscreen) and his body is shown.
There is a small amount of language: "b**ch" is heard once or twice, "a**hole" possibly once, but there is some name-calling such as "coward" and "Stiff". Stiff is a derogatory term for those from Tris's home faction, Abnegation.
This movie had a surprisingly low amount of romance in it compared to others in the genre. Tris (16) and Four (18-19?) share a couple of kisses, but nothing extreme. In one scene, Four takes off his shirt to show Tris the tattoo on his back, and they kiss. Tris mentions that she wants to wait before going too far. The final stage of Dauntless initiation is going into a simulation where you are forced to face all of your fears. Tris is afraid of intimacy, so there is a scene where a simulated version of Four tries to have sex with her. He tries to take her shirt off and push her onto the bed, although before he gets anywhere, she kicks him in the groin and punches him. All of the Dauntless, male and female, share one room, and so one time when Tris is changing, she is shown in her bra for a couple of seconds.
There really isn't any drinking or drugs in the movie, although while reading other reviews, I noticed that some people saw a short scene of the Dauntless drinking. I honestly didn't notice it, so it wasn't particularly obvious.