The first signs of WDW beginning to decline in quality was around the mid-90's, probably around 1993-1996. That I believe is the general consensus many people seem to agree on (give or take a year or so depending on the person you ask). I was just a child at the time and that is indeed when I started to complain about things. The beginning of that really wasn't very long after I first started visiting the place. So absolutely no, it's not just because we're all old or visited 50 or more times. Even at such a young age and without a whole lifetime of visits I was noticing issues. WDW legitimately is just worse now than it was in the past. With any sort of seriousness or unbiased objectivity, I can't imagine anyone being able to truthfully say the parks are as good now as they were in the 80's and early 90's.
Maintenance and upkeep still seemed to be at least somewhat competent until around the early 2000's or so. Not to say there weren't any issues, may have been. But it certainly wasn't even close to being in as dire of a state as it is today. I believe it has been said that the corporate leadership shut down a massive chunk of the maintenance departments at US Disney parks around the time 9/11 happened. Supposedly they did so expecting that Disney park attendance would plummet as people were more afraid to travel and go on vacation after the bombing, so reduced their money spent on upkeep as a compensation for what they expected to be huge losses. Though from what I gather, regarding WDW this ended up not being necessary as the attendance levels were still good. Still i'm quite sure corporate were more than happy not to have to spend money on properly maintaining their parks (though this really came back to bite them in the when multiple deaths occurred at Disneyland due to unsafe conditions from lack of proper maintenance).
But the beginning of bad creative decisions really probably began at some point in the 90's. Someone else that has more inside information on the company shifts in business practices such as @
marni1971. But many people agree that it had something to do with Frank Wells' tragic and sudden death. Allowing Eisner free reign and his ego to grow out of control, making some very stupid decisions that may have permanently crippled the company's full creative potential.
Of the parks i'd say EPCOT was the earliest to start seeing issues. I think one of the first major signs of trouble was Horizons losing its sponsor in '93. After that its operation was sort of intermittent. It closed down for a year or so shortly after that but reopened while parts of the rest of Future World were refurbed or changed. Finally closing down in '99 to make room for Mission Space. World of Motion followed in '96 to make way for Test Track. And then the original Imagination in '98 to be replaced with one of the current abominations. Not to mention the other messes such as the Ellen version of Energy, Wonders of Life being turned into convention space, the '07 overhaul of Spaceship Earth (i'm more forgiving here because at least a large portion of the ride is still intact and they didn't go through with plans to gut it and turn it into a roller coaster as they wanted to in the 90's) etc.
I should mention that I was actually quite young when these declines were happening. About 5-7 years old during the point when Disney World began its decline. I only began visiting WDW in the early 90's in the first place and caught WDW on the tail end of its remaining glory days RIGHT before it began its great fall. So I had really only visited a few times before things started going sour anyways. I basically got a front seat to this rather painful chain of events. And even to the point of view of a young child it was quite clear that WDW had serious issues. 1997 was my final trip as a kid until I finally returned in 2010 as an adult to find it had gotten even worse, particularly the maintenance of the place was in a dire shape. Though again even back then it was pretty clear that things were starting to fall apart.
Probably Splash Mountain, the '94 overhaul of Spaceship Earth and to an extent Tower of Terror were the last really notable genius moves I experienced occurring at WDW (while Splash Mountain was technically already a Disneyland ride before it came to WDW, it gets some extra credit because it substantially improved upon Disneyland's version). Animal Kingdom is a split issue. Wonderful idea and concept, and an absolutely beautiful park visually. But unfortunately it was built too late. By that time Disney World was in its downward spiral and AK ended up being neutered an value engineered to death, opening in an incredibly incomplete state with many of its intended attractions never built. And the ones that were built were generally massively cut down from what they were intended to be (it remains that way until something drastic changes in the company to allow it to expand and flourish). The one ride i'd say ended up being fully fleshed out and awesome was probably the safari. That might be my choice for WDW's last truly great ride they built to this very day. I never visited Animal Kingdom as a kid (I stopped going to WDW the year before it was finished). I only visited AK for the first time in 2010 as an adult.