Your 21 year old most likely does not qualify as a dependent, unless they were a full time college student for any one semester that started in 2020. You really need to clarify this, because if they were a college student, then their earnings don't matter. They could literally earn a million dollars and still qualify as your dependent.
WHen it comes to college students and dependency status, there's really not much room for choice here. So the 21 year old's status really matter.
