Hi Mahjarul,
I am in a similar situation. My brother in law is now 21 and dependent. We applied to the 173 visa but I am afraid by the time the 173 -> 143 visa is processed he will no longer be a dependent child. My gutfeel is that he will no longer dependent and he will have to find alternatives to move to a permanent residence once he's here in Australia. I am happy to hear other opinions in this particular edge case