Edit: Seriously, I don't usually write posts this long...sorry about that. The Simpsons was my greatest obsession as a kid/teenager, so I can be quite passionate about it, which the book I've written below makes pretty obvious.
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I am a 20-year-old hardcore Simpsons fanatic, so I suppose I should share my experiences and opinions here.
I began watching The Simpsons regularly during season 12, which made me around 10 years old. At the time I was in love with it. At first I didn't realize how much of The Simpsons there was, but FOX showed reruns every weeknight, and I made it a point to watch it every single night. By the time the DVD sets started being released (which I now own 1-13 of and have watched numerous times, including with the commentaries) I'd already seen almost the entire series simply through those reruns.
I loved the show for somewhat different reasons than you put forward. Bart was never one of my favorite characters, and although the fact that it was an adult cartoon that appealed to me at first and got me to start watching, it wasn't what made me want to watch it all the time. It was the brilliantly-written conversational humor and cheeky visual gags, the fact that for every hilarious joke there was also a touching moment, the fact that the show touched on issues that even live-action adult shows were afraid to talk about. Those were the things that always got me, even at a young age.
As I continued to watch, I didn't sense too much of a steady gradual decline. Since I first started watching the new episodes of season 12, I accepted that every once in a while one of the episodes wasn't quite as good as the rest, or that some were just downright bad. Since I didn't have any of the DVD sets at the time, I didn't know which of the reruns I watched were from what seasons, but there wasn't a single rerun I saw that I thought was bad.
To me that seems to disprove your "outgrowing" argument, at least for me. Since I was exposed to the older episodes and the new episodes at the same age, and held the same standards for both (I was too young to do otherwise), I can't see how my age or anything else skewed my opinion. Nobody else I knew was a big Simpsons fan, so there was never anybody who told me that the new episodes weren't as good. I just seemed to accept that the reruns were always excellent, and the new episodes were somewhat hit-or-miss, even if I still enjoyed the new episodes.
As I already said, I sensed no gradual decline in quality. I enjoyed the series all the way through season 15 about the same as I always had...in fact, I remember particularly enjoying the season 15 finale. But when season 16 started, something felt entirely different. It felt like an entirely different show. Every episode just seemed like total crap. I continued to watch regularly through the beginning of season 18, but there were only a few (and I literally mean a few) episodes that I thought were any good, and even those I didn't consider great. I thought that perhaps the show was sucking because they were putting all their efforts into the movie, and when I went to see the movie and enjoyed it, I thought that perhaps now the show would get back on track...but it didn't. I haven't watched the show regularly since then, and I likely never will again.
At the time I guess I assumed that I had outgrown the show, but in retrospect I can't see that as being the case. Once I started buying the DVD sets and watching through them, I still loved every single episode (although I feel personally as though it found its real stride around season 5, keeping a consistent peak quality through season 8), even the few ones I hadn't seen before. It was at this point, rewatching them, that I sensed the gradual decline that began for me after season 10, but I still enjoy everything through season 15. At the time I stopped enjoying the show, however, it wasn't gradual...it was literally that season 16 felt entirely different. Looking back at some of those episodes, I can see why. The voice actors approach to their characters seems to change completely. The show feels much more frantic, the pop culture references are far less subtle, the jokes feel way too forced, and the stories feel much less thought out.
I understand that it's entirely possible, hell even probable, that I'm rationalizing my opinion, and that indeed it was the ages I was when I watched that affected my opinion. But I still stand behind my view that from season 16 and onward, the show became not just not as good, but unwatchable.
I don't know how much of The Simpsons you've seen, MovieBob, but if you're basing your "The Simpsons are still good" argument on the basis that most people say that it sucked after season 10 and you still find season 14 enjoyable...perhaps you should be a little more patient and watch further to see what everyone means. There are 23 seasons now...that;s plenty of time after season 14 for the show to start sucking. Still, if you've actually seen every episode of the series and continue to watch the new episodes, and it's your opinion that it still maintains a certain level of quality, then I suppose we'll just have to agree to disagree.
I'm happy to hear your opinions on my favorite show of all time, and am looking forward to seeing part 2.