There's no single national age at which child support ends. Most states use 18 or high school graduation (whichever is later), but the details — and the exceptions — vary enough that it's worth checking your specific state.
The common baseline
A common pattern is that support continues until the child turns 18, or graduates high school if that happens after 18 (common for students who started school a year late or repeated a grade), whichever comes later — but plenty of states have their own variation on this.
Exceptions that extend support
Many states allow support to continue past the usual cutoff for a child with a significant disability that prevents self-support, and a smaller number of states allow (or require, if agreed to in the original order) support to continue through college for a defined period.
It doesn't always stop automatically
In some states and some cases, an order doesn't terminate itself the moment a child turns 18 — depending on how the original order was written and your state's procedure, you may need to file paperwork to formally end the withholding, especially if there are multiple children on one order and only the oldest has aged out.
FAQ
Does child support automatically stop at 18 everywhere?
No — some states tie it to high school graduation instead, some extend it for disability or, less commonly, college, and the exact mechanics of ending an order vary by state.
What if there are multiple children of different ages on one order?
Typically the total amount is meant to decrease as each child ages out, though whether that happens automatically or requires a new filing depends on the state and how the original order was structured.