How young and/or innocent does Tom Robinson act?
The first thing I thought of when I read your question was Tom's failed escape from the prison yard. At first I thought of his unwillingness to believe Atticus that they really did have a chance in the appeal process as an example of Tom's youth and immaturity. But I don't think that was the case...I think he simply had given up hope, and what had happened up until this time to cause this loss of hope? He had been falsely accused of raping a white woman and the jury, despite hearing overwhelming evidence to the contrary, found him guilty. His unwillingness to listen to Atticus was simply a man's giving up on life in the face of overwhelming odds.
So, what could be examples of his youth and innocence? I would say that helping Mayella Ewell in the first place shows his innocence in the ways of life, particularly life in Alabama in the 1930s. He actually was a good man, willing to help his fellow man (or woman), and was not looking for anything in return. He just felt sorry for her, which to the jury, was unforgivable. They simply could not accept that a black man should have anything but Christian charity in mind when dealing with a white woman.
Check the link below for a good analysis on Tom Robinson, as well as the other characters from this book. Good luck!