Foster Youth Dental Act of 2021 This bill expands Medicaid coverage of current and former foster youth, particularly with respect to dental services. Specifically, the bill requires state Medicaid programs to cover early and periodic screening, diagnostic, and treatment services, including dental services, for current and former foster youth under the age of 26. States must set a special payment rate, based on rates in the private sector, for dental services provided to such individuals; the bill provides an enhanced Federal Medical Assistance Percentage (i.e., federal matching rate) for excess expenses that result from such payments. The bill's requirements take effect 80 days after the date of enactment of the bill. The bill also accelerates the application of certain provisions that require a state Medicaid program to cover former foster youth from other states until the age of 26; such provisions currently apply to former foster youth who reach the age of 18 on or after January 1, 2023. The bill instead applies these provisions to former foster youth who reach the age of 18 on or after the date of enactment of the bill.