The book Divergent was written by Veronica Roth.
In the book, the character Four is just 18, but since the actor chosen for the role (Theo James) was 28 during shooting, the filmmakers opted to split the difference and peg him as being around 24.
Uriah is not even in the movie. There is a blink-and-you'd-miss-it moment, and his name was listed on the Dauntless initiates board, but otherwise the film took him out.
In the book, Peter was exclusively violent and unlikable, but actor Miles Teller brought a slightly humored approach to the film character. That's not to say he wasn't still a cruel jerk, but the lack of eye-stabbage and towel-yanking and such did tone down the terror a bit.