It is fairly unlikely for a 12 year old to be reaped, as they only have their name in usually under 10 times. it's even more unlikely for it to happen in a rich district, like district 2 and it almost never happens that no one volunteers in that 12...
"Never underestemate her, she may be young but she's far more vicious than anyone you've ever faced" Haymitch said to Peeta and Katniss as they watched the 12 year old killer on the screen.