As for the argument that we all simply outgrow these things at some point, I dispute such claims as many of the shows I watched as a child turned out to be very intelligent and still appeal to me today while most of the new stuff is dumbed-down garbage. I can't say I have recently sought out children's shows but I have sampled a few out of morbid curiosity or because I fell asleep during Adult Swim and they just don't come close. Rocko's Modern Life and Animaniacs (though not a Nick show) still rank among the best shows I have ever seen and still watch thanks to the pirates at youtube.
The nostalgia factor will always add to enjoyment of something that you otherwise might be less keen on. You gotta ask yourself if you still love every single TV show that you watched as a kid, or if there are a few exceptions that make you say "this sucks".
Like, I loved watching The Adventures of Link and Captain N The Game Master when I was a kid, but I watched some more episodes last year, and I realized they were just terrible. Ditto for the Super Mario Brothers cartoon. Also Teenage Mutant Ninja Turtles.
Yeah, there were kids shows that appeal to adults. There still are. I know plenty of adults who can really get into Spongebob.
But you gotta take a comprehensive look--try looking up weekday afternoon 3-5 PM and Saturday morning 8-12 AM TV listings from when you were a kid--and then try to watch ALL those shows. I'm betting you're gonna find a couple that strike you as utter tripe.