What is the resting heart rate of a 12 year old girl?

The maximum healthy heart rate you should have under high levels of activity is 220 subtract your age, so 208 beats per minute for a twelve year old. The resting heart rate for a twelve year old is within the same limits as an adult, 60 - 100bpm. Resting heart rates vary significantly between individuals, and even over periods of time in a single individual.