When my kids were small, I would ask them every spring, "What do you guys want to do for vacation? Go down the shore or go to Disneyworld?" Each year I would get the same answer 'Let's do the beach again this year and do Disney next year." Consequently, Disney never came.
In 2006, when my kids were 10 and 12, we finally took our much saved for trip to Disney. We stayed at BWV and had an awesome time. Here is my take on age appropriateness.
Older kids have more tolerance and more independence. They can handle the heat better, can articulate their needs better and don't have meltdowns. They can keep up with you and you don't have to have a death grip on them, lest they get lost in a crowd. You aren't tied to nap times and you don't need strollers and diaper bags. Our last trip in 2009, kids were 13 and 15and I could let them go to the pool and arcade by themselves and we spent 2great days at Blizzard beach.
Now the down side of taking kids when they are older is that you miss the whole wonderment aspect. Kids who are amazed at everything they see and little princesses running around after their stay at the BBBoutique. I did miss that but in the grand scheme, if I'm going to spend lots and lots of money on a vacation, I want the kids to remember it and 3-4 year olds aren't going to.
Again, just what worked for my family.