The Affordable Care Act provides for children up to age 26 to be on their parents' health insurance. Before the law, children up to age 23 could remain on parental insurance without penalty. The idea is to make health insurance cheaper for young adults as they enter the work force after graduating college.
In today's economy, it is very difficult for college graduates to immediately land a job that helps them sustain a living on their own. Furthermore, many students are bogged down with hefty student loan payments even if they get a job. Allowing children to remain on their parent's health insurance until they turn 26 gives them more time to get on their feet.