Although Rancho is seen as a college student for most of the film, Aamir Khan who played the character was 44 years old when the film was made. In real life, he is only 6 years younger than Boman Irani, the actor who played "Virus," the director of the college.
Shah Rukh Khan who declined this film to work on "My Name Is Khan" (2010) referred himself as the 4th idiot, who declined to work in such a great movie.
Aamir Khan never stands still in the entire movie. He is always swaying in some direction. Apparently, he concluded that the new generation of kids never stand still after observing his nephew.
The film was shot in reverse, the present-day scenes were shot first and then the college scenes.
Aamir Khan's character Rancho says in the movie that people study engineering, then management, and then become bankers, so what is the whole point of studying engineering. This is exactly what Chetan Bhagat (the author of 'Five Point Someone', the book from which the movie has been adapted) did. He studied engineering from IIT, management from IIM, and became a banker.