I decided to actually look this up. In the novel, Atticus is said to be "nearly fifty," and Jem is ten at the beginning. So Jem was born when Atticus was in his late 30s and Scout when he was in his early 40s.
Now, granted, Ed Harris is 68; Jeff Daniels is 64 (63 when it opened), and Richard Thomas turned 68 yesterday, so all of the stage Atticuses (Attici?) have been cast older. (Gregory Peck was 46 at the time of the film's release.)