Per U.S. government requirements, Children 21 and younger are able to apply with their parents on the same CR-1 or AOS application.
Boundless can help applicants through this process if the child is 18 or younger. Since your child is 20, we would recommend that you seek legal advice outside of Boundless. AILA's Immigration Lawyer Search can help you to find an attorney in your area. U.S. Department of Justice accredits certain non-profit organizations that provide low-cost or free immigration legal services. You can find a directory by visiting Immigration Advocates Network.