The age restriction is listed on many RV parks and resorts but in most they just look at the rig and if its well kept they just let you in with no questions about age. We have an older rig and have never had a problem any were even the upscale resorts. But have seen a couple rigs turned away but their condition really bad. Think its just to give the parks a way to turn down really bad looking rigs.
If your rig is nice looking and well kept I would give it no thought. Odds are it will never come up when checking into any park or resort..