There are many ways to come up with a thesis statement about the theme of coming of age. In light of this, let me offer two possible thesis statements.
In To Kill a Mockingbird, both Jem and Scout are forced to grow up by what they experience. They do not ask to grow up; it is forced upon them.
This thesis statement can incorporate the alleged rape of Mayella. Also the whole trial is a wakeup call for the kids with all the racism and hatred. Finally, there is also the death of Tom Robinson and the attempted murder of Scout and Jem by Bob Ewell.
Here is another thesis:
Scout and Jem are forced to grow up by their experiences, but so were other children like Boo Radley and Mayella Ewell.
This is an interesting thesis, because we are looking at minor characters who suffered while they were younger. What makes it even more interesting is that one grew up to falsely accuse a man; the other grew up to help other children.
In the novel To Kill A Mockingbird, the coming of age and the loss of childhood innocence is an important theme which the author Harper Lee develops using two major characters. From here you can develop the theme using Scout and her brother. Both of these characters face coming of age with the knowledge that the games they played at the beginning of the novel, such as making fun of Boo Radley without thinking about the hurt they caused, no longer fit the people they are now. Their belief that good always wins over evil no longer fits either now that they have seen what happened at the trial. More examples exist about these two characters, or you may choose to expand which characters you include. How many characters, how many examples, and how much development you choose to use depend of the length of your paper.