Varies a little depending on the state, but generally speaking:
18 years of age. May extend past 18 if still in high school. Support ends when the child graduates from high school or reaches 19 years of age. 18 years of age, or as long as the child attends high school on a full time basis or a court order requires the duty of support to continue. Hope that helps.
For specific answer, best to google the question in regards to your state. But it won't be much different. I do think in a few states though it does up to 21 years of age, in certain circumstances.
Doesn't have any bearing though on if the child has a job or not.