Have not read the whole thread...but here are my thoughts....take it they are from a very Disney family. 2 kids, 8 and 4 and we typically go to WDW for 2 weeks in the summer.
1st off: there is no "right age"...you need to know your kid/s. Are they active and like rides? Do they get scared easily by noises or large characters? Can they stand for a while without going bonkers? Do they nap...how are they before and after naps? Are they picky eaters? Walkers or get tiered easily, do they sit well in strollers? All of that needs to be considered before you even attempt to book a trip.
What time of year do you typically vacation? The steep drop in attendance during those "slow times" as become less and less, but traveling in the off-season is better with kids. Generally shorter waits, calmer temperatures, and the parks shut down earlier, so it kind of forces you to get back to the room or resort and have a nice meal, go swim, or put the kids down and spend the rest of the evening with your S/O enjoying a bottle of wine on the balcony.
How tall are they? Do they hit the minimum required heights for most rides? Some kids grow late...if thats yours better to skip this year instead of paying for them with limited attractions to do.
You realize you don't HAVE to go to the parks everyday? Take a day off every 2 days. If you are staying on site, most of the resort pools and play areas are amazing. Also, there are tons of entertainment at the resorts, even available if you are not an onsite guest.
• Take a horse and carriage ride at Port Orleans
• Miniature golf on either of the 2 Mini-golf courses
• Rent a small speed boat at the Contemporary and buzz around bay lake for a bit.
• There are free outdoor movies and a campfire every night at each resort.
• there are babysitting services for hire so you and your S/O can go enjoy a nice adult dinner (we have never done this)
• Join the pirates club for the day—its like a kids day camp with WDW CM guides who take the kids around the resort on scavenger hunts and do activities.
• Hit Downtown Disney/Dis Springs for some shopping...or bowling at splitsville,...or a movie at AMC...or DisneyQuest.
• Go to the campgrounds and visit Circle D Ranch. Its where they house all the horses that are used in the parks. Its free to go pet them and learn from their trainers. There's also a little original Walt history here.
• Off grounds in Orlando there is plenty of things to do
Its true kids won't remember everything about the trip. But just tonight my 4 year old was asking when we were going back so we can ride his favorite attractions.
IMO, there is a very thin slice of time where they are into Princess and Pirates, and once its gone, its gone. Going to the parks with older kids is indeed fun, but its a different kind of fun. Disney does an amazing job with imagination and tapping into kids fantasy worlds, and the looks on their faces as they experience those moments is priceless.
No matter what age your kid/s are....you will have a great time as long as you are honest with yourself, adjust your expectations accordingly, and don't try to push through and do everything...you can't...and it will only lead to your family hating each other and the trip.