Northern Territory DISCHARGE NOT ALLOWED if you are 18 years and over.

Northern Territory, Adoption of Children Act

Part 3–Adoptions Under This Act,  Division 6 — Adoption orders
44 Discharge of Adoption Orders

(2)     The Court shall not make an order discharging an order for adoption under this section if it is satisfied that:
(a)     the child has attained the age of 18 years …