Thanks, Disagreeable, I missed that once I got into the thread.
The reason I asked is that there are a few states where the age of majority is 19, and a few where it is either 18 or graduation from high school, whichever is later. Indiana is not one of these states. Therefore, the 18 year old is already legally emancipated by virtue of her age.
That being the case, jacksgal's answer holds. It is not so much emancipation that you're looking for as it is a modification of the support order. This is something for which you'll need to consult an attorney.