An unmarried child under twenty-one at the relevant time can be a derivative. A twenty-two-year-old is generally too old, though the Child Status Protection Act may preserve eligibility in defined circumstances and should be assessed rather than assumed. A sixteen-year-old has room, but not unlimited room.
Age is measured against a moment, not against today
Derivative eligibility belongs to a spouse and to unmarried children under twenty-one, and the difficult part is identifying the moment at which age is measured. The Child Status Protection Act can, in defined circumstances, subtract certain processing time from a child's age, which sometimes preserves eligibility for a child who has passed the birthday during the process. It is a statutory calculation, not a discretion, and it should be worked through with counsel using the actual filing and approval dates rather than assumed to save anyone.
For a household with a twenty-two-year-old, the realistic planning assumption is that the adult child is outside the case and will need an independent basis to live in the United States, whether study, employment or a later family petition, and that should be discussed openly rather than discovered at the interview. For a sixteen-year-old the arithmetic is more comfortable but still worth doing, because visa availability in some categories can add years.