Shiho Miyano died while taking the APTX. The only one left is Haibara Ai. Even if she turns back, she can't erase the memories of the Black Organization or the APTX, and she won't be able to do that either way. Childhood does belong to a child, but you never had to be a kid to be a 'child'. Right now, I believe I'm more in my childhood then I ever have been- I'm experiencing so many new things, I'm learning so many new things, I'm figuring out how to do new things, I'm getting new friends, and so much more. Childhood is being able to have that period of time where you can be free and just have fun. If Ai could just let go of Shiho Miyano, she could experience childhood. She took the APTX to suicide. She took the APTX to be free. She will be free if she takes her chance. Otherwise, she'll just suffer more pain, and I don't think she's that masochistic.