The Children's Act is applicable to all children and, by implication, parents living in South Africa, so, yes, the principles in that Act will be applicable to your situation.
The Children's Act does not set a specific age at which a child must be heard. The section in the act referring to child participation is section 10 and it reads as follows:
10. Child participation.—Every child that is of such an age, maturity and stage of development as to be able to participate in any matter concerning that child has the right to participate in an appropriate way and views expressed by the child must be given due consideration.
I would argue that a 15 year old would fit the description.
Just note that the best interest of the child principle is still applicable and the choice of the child will merely be one of the factors that the court will take into consideration when making a decision on custody. It may or may not be the deciding factor, depending on the other facts and on the Magistrate/Judge who needs to make a call on this. In most cases where the child is this old, however, it will be a pretty big weight on your side of the scale.
The fact that she has been living with you for quite some time will also count in your favour. Our courts do not like to disrupt the routine and lives of the children under its jurisdiction.