The unmarried nineteen-year-old can be a derivative if she remains unmarried and her age qualifies at the relevant statutory moment. The married twenty-six-year-old cannot; marriage removes derivative eligibility regardless of age. Each child's position should be assessed individually rather than assumed from the household.
Marriage and age each disqualify independently
Two separate conditions apply to a child, and failing either is decisive. Age is measured at a statutory moment rather than at filing, and the Child Status Protection Act can subtract certain periods from a child's age in defined circumstances, which sometimes preserves eligibility for someone who passes twenty-one during processing. That calculation should be performed on the family's actual dates rather than assumed to rescue anyone.
Marital status is simpler and harsher: a married child is not a derivative, whatever their age, and marriage during the process removes eligibility that existed at the start. For this household the practical consequence is that one child is inside the case with an age question to be worked through carefully, and the other is outside it entirely and needs an independent basis if he wants to live in the United States. That is a conversation to have with both of them at the beginning.
Hypothetical example: an investor in a solar-panel manufacturing venture has an unmarried twenty-year-old child and an adult married child. The first review produces a family chart with dates of birth, marital status, and the projected immigration sequence. Civil records settle who may derive status; the petition and visa process dates inform planning but do not change the underlying family rules.
The family should decide early whether the younger child needs an independent alternative if timing becomes tight. A common mistake is treating every dependent child as automatically included, then discovering that age or marriage creates a separate immigration question.