The US federal law called the Individuals with Disabilities Education Act (IDEA) ensures that age-appropriate, free public education is made available to children with disabilities nationwide; this includes but is not limited to special education. IDEA is responsible for governing state and public agency provision of the necessary services, including early intervention. Those eligible range from infants through youth. Part C relates to infants through age 2, and Part B to children ages 3 to 21. Congressional reauthorization and amendment occurred in 2004, and again in 2015 as Public Law 114-95, the Every Student Succeeds Act. Within the US Department of Education, the Office of Special Education Programs (OSEP) administers formula and discretionary grant programs related to IDEA.
In terms of the civil rights of students with disabilities, Section 504 is related to but not part of IDEA; rather, it falls under the Department of Education’s Office for Civil Rights (OCR), as part of the Rehabilitation Act of 1973, and in conjunction with Americans with Disabilities Act of 1990, Title II. OCR is currently enforcing Section 504 regulations consistent with the Americans with Disabilities Act Amendments Act of 2008 (known as the Amendments Act).
The OSEP and OCR official websites contain up-to-date, lucid explanations of the similarities and differences.