Answer:
It is only after puberty that a girl can become pregnant, but if you are asking about the right age, you are correct that having a baby too young can be bad for the mother's (and possibly the baby's) health. While it may not be dangerous, it is certainly unwise and not a good idea for an 11 or 12 year old girl to get pregnant. Their body is not yet entirely developed and it could cause all kinds of health issues.
Further, beyond the health issues, a pre-teen is not mature enough to care for a child. In today's complicated world, it requires a good education and the ability to be a positive role model. Few young teens are ready to devote the time and attention needed to raising a child. Therefore, they are better off waiting until they are much older and better equipped to be a good mother.