When do children have to get their own health insurance?

I'm quite curious about the American health care system and was wondering if someone could tell me what do children do? Can they go on their parent's insurance? Do most people who attend college have health care insurance?
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As long as children are in college, they are usually covered by their parent's insurance (if they have any). Different states have different rules about how old a child, not in college, can stay on their parents' policy. Prior to college, all children are, typically, covered under their parents' insurance. I don't have statistics at hand, but most (i.e., more than half) of our Nation's college students have health care insurance, almost all of whom are under their parents' policies.
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