Crazy Heart

Otis "Bad" Blake (Jeff Bridges) is a 57-year-old alcoholic singer-songwriter who was once a country music star. He now earns a modest living by singing and playing his guitar in small town bars across the South-Western United States. Having a history of failed marriages (four that he admits to, although a reference is made to a fifth he does not discuss) Blake is without a family. He has a son, aged 28, with whom he has not had contact in 24 years. He is mostly on the road performing, staying in cheap motels and travelling in his old '78 Suburban alone. The film opens with his arrival at a bowling alley for a show.
Enter Jean Craddock (Maggie Gyllenhaal), a young journalist after a story, divorced and with a four-year-old son, Buddy (Jack Nation). She interviews Blake, and the two enter into a relationship. Jean and her son become a catalyst for Blake beginning to get his life back on track. In doing so, he lets himself be pushed into renewing a professional relationship with Tommy Sweet (Colin Farrell), a popular and successful country music star he once mentored, and plays as the opening act at one of Tommy's concerts, despite his initial balking and wounded pride at being the opening act to his former student. He asks Tommy to record an album with him, but Tommy says his record company insists on a couple more solo albums before a duet project can be recorded. He instead suggests that Blake concentrate on writing new songs that Tommy can record solo, telling him he writes better songs than anyone else.
Blake's drinking soon gets out of control, and he ends up running off the road while driving drunk. In the hospital, the doctor informs him that although he only sustained a broken ankle from the crash, he is slowly killing himself, and must stop drinking and smoking and lose 25 pounds if he wants to live more than a few more years. Blake's relationship with Jean makes him start to rethink his life. He calls up his son to make amends, only to have his son tell him that his mother, Bad's ex-wife, has died, and hang up on him. The relationship starts to look up, with Jean visiting him with her son Buddy. After a situation where Blake loses Buddy briefly at a shopping mall while drinking at a bar, Jean breaks up with him.
After losing Jean and her son, who were becoming his only family, Blake resolves to quit drinking. After going through a treatment program at a rehab center, and with support from his Alcoholics Anonymous group and his old friend Wayne (Robert Duvall), Blake finally manages to get sober. Having cleaned up his act, he tries to reunite with Jean, but she tells him that the best thing he can do for her and Buddy is to leave them alone. After losing Jean, Blake finishes writing a song that he thinks is his best ever, "The Weary Kind", and sells it to Tommy.
Sixteen months later, Tommy plays "The Weary Kind" to an appreciative audience while Blake watches backstage, as his manager presents him with another of the large royalty checks for the song. As Blake is leaving, Jean approaches him, saying she has come to the show as writer for a large music publication. As they catch up, Blake sees an engagement ring on Jean's finger and tells her that she deserves a good man. He offers her the money from that royalty check for Buddy to have for his 18th birthday, which Jean initially refuses but eventually accepts after Blake says the song would not exist without them. Jean asks if Blake would like to see Buddy again, but Blake declines saying it might be too unsettling for the boy. The film ends with them walking away from the concert, chatting with each other.

Question: How old is Jean's son?
Answer: four