The 25-year exception is intended for collectors and restorers. It is aligned with many states allowing historic/vintage/"horseless carriage" plates with limitations and reduced fees for vehicles over 25 years old. These rules are often something like must be driven < 1000 miles per year except trips to/from exhibitions and shows. The federal standards don't get so specific about restrictions on collector vehicles, they just get a lot looser about what can be registered after 25 years.