The quick answer is no. “Grandfathering” does not exist in the ADA. Since the ADA was passed, there has been a barrier removal obligation. The trigger to remove these barriers is the fact that they exist. It does not matter how old the building is. There are three reasons that barriers do not need to be removed.
1. It is not readily achievable to do so.
2. It is technically infeasible to do so.
3. It is a qualified historic building.
While all this sounds simple in concept there are a lot of complexities to making these determinations.