How old is Tom Robinson in To Kill a Mockingbird?

Tom took the oath and stepped into the witness chair. Atticus very quickly induced him to tell us [sitting in the courtroom]:
   Tom was twenty-five years of age; he was married with three children; he had been in trouble with the law before: he once received thirty days for disorderly conduct. (Chapter 19)

Tom is in the courtroom being questioned by Atticus, his defense attorney, when we learn how old Tom is. The above quotation gives Tom's age as twenty-five. For a young man, he already had a wife and three children, a fact later brought forward by the prosecuting attorney as a challenge to the truth of Tom's testimony: if a young man of twenty-five had a full day's work to do, had a wife and family to go home to and had chores of his own at home to do, what was he doing stopping to do chores for Mayella, a white woman with a father and brothers and sisters at home to give her help if it was needed? 

An interesting consideration when thinking about Tom's age is that, for a little more than half of his life, he has had one arm that is useless, a point Atticus brings up in his defense of Tom, because when a boy still, Tom's left arm was caught in a cotton gin and crushed, leaving it in tact but useless. So for approximately twelve of his twenty-five years, Tom has not had a functioning left arm; he has had only one arm that functions, a fact Atticus uses to illustrate that it would have been impossible for Tom to leave two-handed markings on Mayella's neck.

Thomas Robinson reached around, ran his fingers under his left arm and lifted it. He guided his arm to the Bible and his rubber-like left hand sought contact with the black binding. (Chapter 19)

In the scene in which we are told Tom's age, Mayella has already testified about Tom and, as Scout puts it, "looked at him as if he were dirt beneath her feet" when she left the witness stand although it was she who lied about him, not he who lied about her; to the contrary, Tom had befriended Mayella when no one else would. In a compliment that bespeaks Tom's greatness of mind and heart, Scout compares Tom to Atticus, even though Atticus is a much older man than Tom's twenty-five years, by saying that Tom's manners were "as good as Atticus's" manners. Considering that To Kill a Mockingbird is a tribute to Atticus, the protagonist, as well as a tribute to the entrapped victim, Tom, to equate these two men--a young man of twenty-five with woefully limited experience and a mature man with broad experience and education--makes a significant comment about the protagonistic qualities of Tom and, in essence, makes Tom a second protagonist.

While these two men are favorably compared to each other, the difference between the two men goes beyond age, education and experience to encompass ethnicity, sociocultural "psychosis," as historian Paige Smith called it, and socioeconomic barriers. Because of Atticus's position in the society, culture, economy, and race divisions, being entrapped by Mayella--had he been--may have resulted in her doom: she would have born the brunt of her folly in profound ways. In contrast, because of Tom's position in society, culture, economy, and race divisions, being entrapped by Mayella results in his doom in the most profound way. This difference between these otherwise comparable men--racial, social, cultural, economic difference--is what twenty-five year old Tom was feeling when he said, "if you was a nigger like me, you'd be scared, too."

"Why did you run?"
"I was scared, suh.
"Why were you scared?"
"Mr. Finch, if you was a nigger like me, you'd be scared, too."