Process manufacturing plants have a tendency to keep automation systems and associated networks up and running for decades. Some estimates suggest that easily half of the automation systems controlling North American plants have parts that are at least 20 years old, and 30-year-old systems are not rare.
Those systems do not look like they did when they were initially installed, however. Hard-disk drives, monitors and keyboards that receive constant use simply do not survive over decades, and such old equipment is not available anymore. If you ask your local computer store for an IBM XT motherboard and 20 MB hard drive, you will get some puzzled looks. So, unless your automation systems are brand new, they are likely multigenerational due to obsolescence and devices wearing out.