A spouse and unmarried children under twenty-one obtain conditional residence as derivatives. Adult children and parents are not derivatives and would need an independent basis of their own.
The age question needs a real calculation
Because these cases run for years, the age of a child at the moment of filing is not the only date that matters. Statutory provisions exist that can, in defined circumstances, protect a child's age during specified periods of processing, and their application depends on the facts and on visa availability. That calculation is technical and should be done by someone qualified rather than estimated.
Families with a child aged sixteen or older should ask for it at the first consultation, because discovering a problem at the visa interview leaves very few options. It is also worth noting what derivative status does not include: parents who would move with the family, and adult children, are outside the petition entirely and need their own route.