Most likely because this answer is very wrong. You rarely have situations in developing countries where the parents are supporting adults in their mid 20s, and it's almost always the other way round.
The correct answer, as others have pointed out, is the informal economy, with them taking up a bunch of odd jobs one after the other.
To focus on a subset of 15-29 year olds, what do you think 15-18 year olds are doing without either being in school (which is free in South Africa) or holding jobs (which would probably be illegal)? Most likely, supplementing the family income by taking up jobs in the informal economy.
It does indeed happen in the developing world that adults in their mid 20s are supported by older members of their family, it just might not be their parents. In West Africa, for instance, it is common for entire families to be supported off of a couple of lucky relatives who managed to get to Europe.
The correct answer, as others have pointed out, is the informal economy, with them taking up a bunch of odd jobs one after the other.
To focus on a subset of 15-29 year olds, what do you think 15-18 year olds are doing without either being in school (which is free in South Africa) or holding jobs (which would probably be illegal)? Most likely, supplementing the family income by taking up jobs in the informal economy.