It's never outright stated how old the creek elders actually are. But based on the namedrops Elder Barry did in the episode "Kelsey the Elder", the earliest being "Pirates of Dark Water" and "SWAT KATS" both of which came out in the early 90's, 30's wouldn't be too far off. Then again, that's not to rule out that they didn't simply watch them via VHS or DVD later on. I certainly never watched SWAT KATS when it debut (considering it came out 3 years before i was born) but stumbled upon it via Boomerang reruns during the 2010's.

I don't think the creators intend to group 30 year olds with 10 year olds, so they probably sit at a comfortable "nearing 20", and all those shows were just to perk the ears of the actual adults who watch the show.