Each of those themes make up the entire novel, so that is a HUGE question to answer.
Race was discussed with the trial, showing how awful/racist most of the town was towards Blacks and Tom. The good people like Atticus, Miss Maudie, Heck Tate and the judge kept the children aware of how to truly treat others, no matter what the color of their skin is.
Class was discussed with Aunt Alexandra and how she treated the Cunningham family. She was so concerned with a person's upbringing and heredity that she didn't take into account how Scout "Fine Folks:" they were people who did the best they could with what they had.
Gender wasn't discussed much, except for Scout living in her overalls while most girls were supposed to be in dresses. Many times she was made fun of from both kids and adults for how she dressed normally. Miss Maudie served as a mix of the two. She was a gardener most of the time outdoors, but cleaned up nicely and appeared queen-like on her porch each night.
The becoming of age was mostly Jem's maturation. We see that through Scout's eyes. He is able to see that Boo is a good person and is only trying to be friends with them. He is so upset with the trial's verdict when he KNEW Tom was innocent. We also see how Scout matures in the very end. She is finally able to see things from others' perspectives. Mostly we see this when she is standing on Boo's porch in the second to last chapter. She realizes what she and Jem meant to Boo--"his children"--and she understands him so much more. She shows her understanding when discussing "Stoner's Boy" with Atticus in the very last scene of the book.