As the owner, developer, rules maker of a RV park (not a resort) I have the 10 year rule as a benefit for my other site renters. It costs me money when I turn away a POS and yes I do not ask a well maintained rig how old it is, even when I know it's a few years more than 10 years old. If you have a nice looking 12-14 year old rig I would not worry about it's age.
As the owner of a Airstream I know how ageless they can look. I pained me recently when I had to say NO to a nice couple with a nice looking Airstream motorhome... it was 22 years old. If you bend a rule that much, someone else is going to make you regret being nice when they say "well, you let them do xyz".
"If a park has age restrictions, they don't want your business anyway. go somewhere else." I'm pretty sure I'll be glad you went elsewhere... it's still a free country.