
Skip Beat, One Piece, Case Closed/Detective Conan (especially this one but it is so episodic, you can almost pick up any volume and understand that volume)
...Pokemon had been there for a very, very long time. I wasn't even twelve when Ash first took his journey but I was closer to three times that age than twice that age when they finally decided Ash had done too much in 1-2 years.
DragonBall was originally made as a joke and look at how long it has been going on and the spin offs plus the remake (which was needed because the original anime had caught up to the manga's plot and need to "power up" to use up time.)
An American example would be SpongeBob SquarePants. I don't remember if it's finished but I remember when the first season was coming out. Or the Simpsons (which I am more certain ended already.) South Park is also getting up there.
The other bits of media that have gone on long enough their target audience has aged out have been programs like Sesame Street, Mr. Roger's Neighborhood, Reading Rainbow, aka, shows designed for their audience to graduate from watching them. The difference is programs like these didn't have the budget or time to spend on dragging out their message (which is also why it is hard to get a complete collection of any of them.) The creator of One Piece said in an interview about a dozen years ago he had gone through about 70-ish percent of the plot and a more recent interview had him barely move that percentage.

One Piece author keeps telling the voice actor of Luffy “Just ten more years”. The woman’s gonna die before the series ends at this rate. Honestly it’s something she worries about. She’s even said in interviews, “If I step down now would the fans find that easier to accept?” That series is probably gonna run until its creator dies. Even if it ends, it won’t end. It’ll be like Naruto and Boruto.

Yeah, I never got into One Piece and because of this, I never will. All I see is a good start with a drawn out plot. I learned that lesson with all the other anime series I mentioned (and Naruto. I stopped reading and watching around the same area.)
There are some series that never got more than 12 episodes I believe deserved more but even back when most anime went for 26, there was usually a filler episode.

Haha yeah. The Naruto anime was especially bad for that. The plot was only there to get you from filler to filler. Bleach too. For that series I used to get frustrated feeling like, “author-San is it too much to ask for the plot to make even a cameo appearance now and then?” And DBZ? The fight scenes were always at least 5 times longer than any appearance of plot. Most of all, I hate tournament based series where the plot is basically just a fringe wrapping. Well, as I said before, welcome to Asian graphic novels +anime!
I started reading this when i graduated from high school. I’m about to graduate from college and it’s still going strong