Are older backflow devices grandfathered-in because protection was not required when my home was built?

Each state has different requirements.  You should review your state's plumbing code to determine what is in or out of scope.

For instance, the State of MN requires all High Hazard devices (RPZ) to be tested annually, regardless of the date of install. Regarding all other testable devices (PVB, SVB, and double check), these devices need to be tested annually if they were installed after January 23rd, 2016. Water municipalities do have the choice to go above and beyond the State’s requirement to test all testable devices regardless of the date they were installed.