A twelve year old is a child. They know right from wrong but do not have the ability to support life on their own if they were left in the street. This means that they are not grown. They can't get a job or a house. They can't make money to buy food. They don't have land to grow food.
In nature most animals start their adult lives at sexual maturity, and human history shows that this was the natural course for humans as well. While 18 is probably a little old to be just starting to be considered mature, and maturity rates vary between individuals, there are few if any 12 year olds that are sexually mature, so a 12 year old would still be considered a child.