The Americans With Disabilities Act (ADA) was signed into law in July 1990. 

It is a civil rights law introduced to prohibit discrimination on the basis of disability. It applies to places of public accommodation, commercial facilities, and state and local government facilities.

Appended to the ADA was the Americans With Disabilities Act Accessibility Guidelines,

In 2010, an updated version was introduced and approved by the Justice Department — ADA Standards for Accessible Design (ADASAD).

While many of the components of the ADAAG and ADASAD have been incorporated into building codes it is important to note that the ADASAD is not a building code but a standard by which a facility that is open to the public may be determined to be accessible.

The International Building Code (IBC) is a model code that is adopted across the country and applies to new construction, the ADASAD can be applied to any existing structure to confirm accessibility.

Older facilities are not exempt from the provisions of the ADA and there have been numerous civil suits for discrimination that have been filed across the country.

The New York Times published an article on this back in 2012.

Given that the ADA was created in 1990, the courts are not looking kindly on businesses that claim ignorance of the law.