Family unification - joining dependent child

Dear All
Most countries have a provision for family unification in their immigration law. A child also has a right to a family life, according to UN.
I note from the thread on Domicile, that if you are married to a Namibian citizen you have right of domicile, and if you have right of domicile, so do your dependent children.

1 If and when I obtain right of Domicile (or residence), if a child is over 18 but a full-time student, is he considered a dependent child? If so, until what age?

2 I have a child (6 years old) who is a Namibian citizen and lives there, and who is dependent on me (my name is on his birth certificate). In four years I plan to join him and settle there. Does the right of domicile work this way round? Or do I have to go through the whole (unlikely) residence process? What section of the law or course do I have to take to join him there.

Many thanks for your help.