Dawson Casting: Russell Crowe was in his late 30s during the filming, which begins as Nash enters Princeton University. While his age is not stated in the movie, in reality this happened when Nash was 20 years old.
It's impossible to avoid Dawson Casting when a character is played by a single actor over a timespan of about fifty years.
However, most makeup and CGI artists will tell you that it is much easier to make someone appear older than they really are than younger.
Throw It In: Nash's rambling about different kinds of tea came from the man himself during one of Russell Crowe and Ron Howard's conversations with him. As they left, Crowe said to Howard "I'm using the tea shit."