Mark Wahlberg was told by the filmmakers that they wanted him to portray the 'older' guy. Initially Troy actor, Garrett Hedlund was considered for the younger role but later Justin Bieber was signed.
“I get this call from Paramount (where they tell me about the story about) an old guy and a young guy. I'm like, 'Cool, let's get Jack (Nicholson). Let's get Robert De Niro. Let's get Robert Duvall.' And they go, 'What about Garrett Hedlund?' I'm like, 'For what?' They're like, 'For the young guy," MTV news quoted Wahlberg as saying.