We stayed at a park in Moab, Utah. One half was very high end and had a two tier age restriction. Motor homes 10 years old or less, Trailers 5 years old or less to get a reservation. It was one of those that "sold" sites in the park (perhaps on a time share basis, but I cannot recall) and rented out sites for the "owners" when they were not occupying them. It was all concrete and very well landscaped.
I see this as a method of making it "easier" to keep unsightly rigs out of the park, avoiding telling an owner his or her unit does not measure up to their face after they pull up needing a site. It also makes the policy somewhat more consistent, less arbitrary and avoids "eye of the beholder" issues.
Needless to say, we stayed at the lower end section of the park, which was still nice. Our rig at the time was 8 years old, though in good condition.