What are three specific scenes in To Kill a Mockingbird where the theme of growing up comes through clearly?
The Closing of the Secret Knothole. When Jem discovers that Nathan Radley has cemented the knothole from where the children communicate with Boo, he assumes there must be a logical reason for it. Nathan tells Jem the tree is diseased, but Atticus points out that the tree looks perfectly healthy. Jem is crushed: He comes to the realization that Nathan has lied to him and that the knothole has been deliberately closed to prevent further contact between Boo and the children.
He stood there until nightfall, and I waited for him. When we went in the house I saw he'd been crying... but I thought it odd I had not heard him. (Chapter 7)
The Missionary Circle Tea. Scout hates the idea of becoming a lady--"I was more at home in my father's world"--but the combination of the proper ladies at the missionary circle tea and Atticus's announcement of Tom Robinson's death puts Scout on her best behavior. She sees that growing up--and becoming a lady--is inevitable:
There was no doubt about it, I must soon enter this world... (Chapter 24)
so, she wisely decides to emulate Aunt Alexandra and Miss Maudie instead of the other hypocritical women guests.
After all, if Aunty could be a lady at a time like this, so could I. (Chapter 24)
Boo Appears. Scout has long given up her "fantasy" of one day meeting Boo Radley in the flesh, but her dream becomes reality when Boo rescues the children on the way home from the Halloween pageant. Boo has been keeping a watchful eye on "his children," has saved them from Bob Ewell, and now stands before Scout. She understands and even agrees with Heck Tate when the sheriff decides to lie and call Bob's death self-inflicted: Boo's privacy will be protected, and Scout recognizes that involving Boo would be like harming an innocent songbird:
"... it'd be sort of like shootin' a mockingbird, wouldn't it?" (Chapter 30)
Looking out over her neighborhood from the Radley porch reminds her of how much she and Jem have experienced during the past two years, and a tired Scout suddenly feels all grown up.
I thought Jem and I would get grown but there wasn't much else left for us to learn except possibly algebra. (Chapter 31)