There may or may not be consideration by a magistrate for actually choosing then parent you can live with. In many cases the only real time a child can choose is that age when they become emancipated.

Here is what I found on emancipation:

According to the Martindale-Hubbell Law Digest (1994), in South Africa
The age of majority is 21, although the Supreme Court may grant
Emancipation to minors at least 18 years of age who are able to convince
The court that they are "fit and proper" to manage their own affairs.


Ref:

Law-Lib: Re: South African Law