Though the CHINS case should be dismissed on your 18th birthday, the warrant may stay in the state Warrant Management System until it is recalled. You will want to be in touch with the attorney who represents you on the CHINS case and make arrangements to go into court to have the warrant recalled.
Until the warrant is recalled, you run the risk of being held for the next court day if you are stopped for a traffic violation or anything like that.
See my blog post about the law that replaced CHINS on November 5, 2012, which I have linked below.
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There is nothing that says a warrant expires on its own. People with 15 year old warrants get picked up. So, if you get stopped by a police office and he/she looks you up from the cruiser and sees that you have an open warrant, you will likely get arrested. Once you are 18, you can think about clearing the warrant yourself by getting to court and explaining it to a judge. You might want to consult and attorney first.