Jesse Eisenberg and Andrew Garfield were both 26 at the time of filming, playing 19-year-old Zuckerberg and 21-year-old Saverin respectively; though they also play the characters a couple years down the line (2007?) in the Framing Device.
Peter Thiel, around 37 at the time he made the angel investment in Facebook, is played by Wallace Langham, who was 44 or 45 at the time of shooting.
Dyeing for Your Art: Justin Timberlake lost 20 pounds in order to look younger, Max Minghella tanned his skin to play an Indian-American, and Josh Pence and Armie Hammer (the Winkelvi) were sent off to a sort of boot camp in order to train as rowers and learn each other's physical mannerisms.
American-born and UK citizen Andrew Garfield playing Brazilian-born Eduardo Saverin.
Indian American Divya Narendra was played by English actor Max Minghella, who is of Italian, Scottish, and Chinese descent, not without minor controversy. The DVD Commentary with the cast stated that Minghella was artificially tanned for the role, which lead to a hilarous moment when Armie Hammer and Josh Pence met the real Divya Narendra. They decided to introduce him to Max Minghella and convinced him (as a joke) to speak in a thick Sim Sim Salabim accent. Upon hearing him, Minghella turned white and spent the next 30 minutes apologizing profusely for his performance.