Parks that have a 10 year rule are trying to avoid having old broken down units on the premises. They normally make exceptions based on appearance, but may not be interested in long-term residents. The age of a unit has less to do with how well it holds up than how the owner has cared for it does.
When buying a used unit one of the most important things is checking the ceiling & walls for any evidence of water leaks ...inside cabinets, closets, etc. Water marks, water stains, bubbling wall coverings, soft spots in walls or floor, mold ...all are danger signs. Broken appliances, etc can normally be repaired, but structural problems & water damage can be very difficult or impossible to repair.