Supernatural: It's Time To Hunt

Alice Foster lived in an orphanage for 15 years, until she found out how her parents really died. Back when she was barley a year old, her parents died in a "car crash." When Alice decided to inspect the situation and all the possibilities she came to the strange conculsion that her parents car was crashed by a possessed human. She interrogated the man who was possessed and he said he remembered the demons name, Azazel. So Alice and of her lifelong friends from the orphanage, Mabel and Finnick, decided to go hunting. After some research on how to kill demons they discovered a whole other network of monsters. They found fellow hunters and learned the things important for killing any monster. One day they met a hunter, he told them that Azazel was already killed, by Sam and Dean Winchester. Although their main goal was broken they continued to hunt monsters of all sorts trying to protect families from the dangers they have received. Now they are 24 and still hunting, and they just made some new friends.

Dean is about 29 and Sam is about 25 in this book.