The income test for a child age 19 or older who is not a full time student is simple, $5050.
There is no income test for a child between the ages of 19 and 24 who is also a full time student, there is a support test. The support test asks, does the child pay more than half of their own support. The child's support includes their living expenses (room and board), clothing, travel, entertainment, medical and tuition. And it's not about income, but about how it's used. Your child could (in theory) earn $100,000, but if they invest it all, and you pay their room and board, tuition and other expenses, then they are not providing more than half their own support. (Of course, if the IRS sees that kind of income, they may ask you to clarify.)
For a child in college, tuition loans in their name counts as support they provide to themselves, since they have to eventually pay it back. You may provide cash support to your child in the form of tuition, room and board, or health insurance from the school. You can also count a percentage of your household expenses (rent or mortgage, food, utilities) as support you provide to your child if your child still has their permanent home in your home. Because of the tuition loan issue, it is possible for a child to "provide more than half their own support" if they have substantial loans, even if they also have support from their parents. It's something you have to add up and check.
