I have never found a state park, county park, FS park, BLM campground, NPS,county fairgrounds, or COE campground that has age restrictions.
So, in oregon that gives you HUNDREDS of campgrounds close to virtually anyplace you want to go.
I know for a fact that NO oregon state park has any restriction on age. However, they do fill up during the summer, especially near the coast so plan on making reservations NOW especially for weekends.
And, in reality we have found very very few private parks with age restrictions on units.