It's up to the age of 23 if they're going to school full-time. After the age of 23, the only time they're eligible is if they're totally disabled and dependent. You could have a 50-year-old child if that person is disabled and totally dependent on the parents. Other than that, if they're not going to school full-time, then it's up to the age of 18.
On March 9th, 2009. See this statement in context.