As you can see by by signature, we've been bringing the kids to WDW since they were born. Every age has its benefits and downsides but I wouldn't trade one single trip. Yes, babies have strollers & bottles but they also don't cost anything, complain or walk away since they are in the stroller. An added plus is that I was able to get both of my kids 1st haircuts at WDW. For many people the biggest reason they stay away is because the child won't remember it. I know they don't remember any of it...but I do
As my kids became more self sufficient they also became more opinionated and lost some of the belief in the magic. When my kids were really little they actually believed the characters really lived a WDW

. Now that DS is 8 & DD is 7 they 'know' that its people in costumes. This doesn't keep them from having fun but I do miss those days when the kids believed in everything.
Now, both kids are concerned with not being treated like a baby. DS runs away from the princesses and getting him to a breakfast requires a compromise and bribery

DD no longer wants to take time out of her day at MK to visit Ariel and doesn't want to dress up any more

On the flip side everyone is tall enough for everything. The rides don't scare them anymore so Mommy isn't left out with DD as much. No-one is carrying anyone any more. They have their favorite restaurants & rides. We can actually have fun with our kids and get to enjoy their personalities. But the biggest plus of all is our shared experiences.
Like the other posters said, I guess the best time to go has everyting to do with affordability. I have always taken the kids because I knew I'd find a way to get there every year. But, if I had to choose an age for a once in a lifetime visit it would probably be more selective about when. In my opinion it was my favorite when my kids were 4 & 5. They were old enough to walk yet young enough to want to sit in the double stroller. We didn't need bottles and they ate whatever I told them to eat. They still believed in the characters but preferred the safety of Mommy's hand to wandering off.
Whatever you choose will be fine. In my opinion there is no such thing as a bad age for WDW, its all about preference.