Quick answer
There is no single support-ending age for NJ, NY, and DC. The order, local law, disability, education, and other circumstances matter.
Do now
Read the termination language in the actual order. Obtain any agency termination or continuation notice. Ask whether an application is needed and whether arrears remain. Gather school and disability information if relevant.
Watch for
Ending future support does not erase an unpaid balance. Do not stop a wage deduction yourself or treat a birthday as an automatic cancellation in every case.
Official sources
NJ Courts — FD modification application packet, NY Courts — Starting a Case in Family Court, DC Courts — Parentage and Child Support Branch.