OP:
Do you recall negotiating with your ex's atty to deviate from the standard (of 18/19 school) when calculating child support. Ck your judgment.
It's very rare, but happens, very infrequently.
If DCS is enforcing the order, they will have the correct date to term - ck with them (they must have the order, to enforce, ergo, they will not term the garnishment too early and leave OP incorrectly in a non-paying state).
PS: As stated, you should stop this order only by a notice from Child Support or by asking the court (via an Order To Show Cause) to issue the judicial council terminatin of wage assignment form to your employer. DO NOT just tell/convince your employer to stop the garnishment.
It would be very unlikely that the court, (reading your judgment & order for cs) or DCS, would terminate your CS too early (more likely they'll keep reducting), but as the prior poster appears to think it is likely, check your judgment for the wording as to when CS stops.