A 12 year old mind which would never remember the specifics of what you did to her. EVERY NIGHT could be like the first time, except you'd get to know her better every time.
You'd learn her limits, her likes, what turned her on (but which she couldn't admit to liking)... you'd soon know her body's reactions better than she would.
You'd know what lines worked on her, how to convince her to go past whatever limits she had, how to please her, and how to instruct her to please you.
Oh god, I want a brain-damaged fucktoy.