You haven't met every 12-year-old. I have a daughter that fit his description, both body and mind, except she wasn't seeking a boyfriend. At 12 she was taking college classes, getting only "A" grades. Nobody suspected a thing. All her friends were much older than her, and they all assumed she was much older. This was without make-up even. Her behavior is more adult-like than that of a typical adult. She gets annoyed by the childish behavior she sees in many adults.

You might even know somebody like this. Unless you demand a photo ID to verify the age of everybody you meet, you just can't be sure.