Child support is the periodical payment from one parent to another to provide financial care for the child and generally ends at the 'age of emancipation' of the child. In most states, the age of emancipation is 18 or 19. Many states have laws that allow disabled children to continue receiving child support into adulthood. What exactly constitutes 'disabled' varies from jurisdiction to jurisdiction. If you are unsure about the law in your state, consider scheduling a legal coaching session with an experienced divorce lawyer.
Standard child support calculations rarely meet the short-term and long-term needs of a special needs child. It is important that you consider all current expenses and the ones that you can anticipate. Make a chart and include categories like equipment, medication, supplements, dietary costs, respite care, modifications to your home environment, and sensory items. The bottom line is, there's only so much money to go around, so it's going to be important to prioritize what is most important (not easy, I know!) Now is a good time, if you haven't already, to maximize every possible benefit your child may be entitled to through grants, government programs, schools, etc.