A 12 year old may be able to get a six-pack with intense physical exercise and a diet specifically tailored toward building muscle mass - however, due to hormonal differences between 12 year olds and adults, building significant muscle mass is not possible for the majority of children.
There are two hormones that are essential to building muscle mass - testosterone, and human growth hormone. Both of these hormones are present in adults. Children of both sexes have testosterone and human growth hormone, but these hormones are present in much smaller amounts than in adults. As detailed on Scooby's Workshop, between the ages of 9 and 13, the level of testosterone in a young boy increases from around 10 percent to 75 percent of the maximum amount of testosterone that he will have as an adult. According to Kid's Health, a diet rich in protein can help children gain muscle strength, but it is difficult to gain muscle mass at this age. Besides the lack of testosterone, the child's skeletal structure will still be developing and growing until later in his teens, so significant weight training can strain and even damage a still-growing skeleton. The optimum period for gaining muscle mass is between the ages of fifteen and twenty-five.