In US immigration law, a child is unmarried and under 21. On the 21st birthday, a child on a dependent visa (H-4, L-2, J-2, F-2, E) stops being a dependent. This is called aging out.
The dependent status ends on the birthday. The young person needs another status to stay, or must leave the US.
A work permit based on dependent status (for example an H-4 or L-2 EAD) also ends.
Getting married before 21 also ends child dependent status.
Change to F-1 student status. Many teens apply to college and file for F-1 before turning 21. File the change with Form I-539 or travel and get an F-1 visa abroad. Start at least 6 to 12 months early. As an F-1 student, international tuition rates and work limits apply.
Get their own work visa, such as H-1B, if they have a degree and an employer.
Another status that fits, such as through marriage or their own green card case.
Talk to an immigration attorney or the college's international office around age 19 or 20.
Green card cases and the CSPA
If the family's green card process is under way, a law called the Child Status Protection Act (CSPA) may "freeze" or reduce the child's age for that case. Sometimes a child who is over 21 in real life still counts as under 21 for the green card. The math is complex; ask your attorney to calculate it.
Waiting for the green card and not planning a backup. Processes can take many years.
Thinking the I-94 date protects the child after 21. The status still ends at 21.
Starting a job on a dependent EAD after the birthday.