What is supposed to happen when a child attains the age of emancipation (or is emancipated under one of the other rules set out in the statute) is that the custodial parent is supposed to notify the other parent that the child is now emancipated. There's a form for that. Custodial parent files the form with the court and sends a copy to the other parent. No problem.
If the custodial parent doesn't send the form to stop the child support, the paying parent can fill it out himself and send it in to the court. The paying parent then has to have the other parent served before the court can do anything with it - like stop the child support, for instance.
Here's where the plot thickens.
Imagine, if you will, that you are the parent receiving child support and you don't really want the child support to end. You know that the court will tell you to pay back the money that you were overpaid, but, since you don't have any money or assets (lawyers call these folks judgment proof), you don't really care.
For instance - child has now passed his 21st birthday. Actually child passed the 21st birthday a long time ago. And Mom is not about to file the affidavit of termination because it would terminate the child support payments and she rather likes having them come in. So Dad fills out the forms to stop the child support and sends them in to the court for processing and to get Mom served.
Mom ducks service. For a long time.
Dad has process servers stake out Mom's house - to no avail. (Mom's really good at ducking service.)
So, how does Dad get the child support stopped? That's the problem - there is no way that's been set up by the legislature for Dad to be able to stop the child support - even if the child won't see the sunny side of 25 again.
Now you might think this would never happen. You would - unfortunately - be wrong. It happens. Some of the judges will help lawyers do a work around - and they certainly deserve praise for their efforts. One would think, however, that a member of the legislature could put a bill in the hopper to plug this rather ugly hole in the statute.