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If she is still in high school, then her benefits will continue, so no her benefits will not stop at age 18. From SSA:"Three months before your child’s 18th birthday, we’ll send a notice to you letting you know that benefits will end when your child turns 18. Benefits don’t end if your child is a full-time student at a secondary (or elementary) school. If your child is younger than 19 and still attending a secondary or elementary school, they must notify us. They must complete a statement of attendance certified by a school official. The benefits will then usually continue until your child graduates, or until two months after reaching age 19, whichever comes first." See SSA link: https://www.ssa.gov/pubs/EN-05-10085.pdf
See link for statement of attendance that you will need to complete and return to SSA: https://www.ssa.gov/forms/ssa-1372.pdf
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