I've traveled to Walt Disney World many times with babies and toddlers. My oldest, now 9, we took as a toddler twice- when she was 2-1/2 and 4. My youngest, now 4, has gone at 9 months, 20 months, and 2 months shy of 3. And honestly, it really wasn't that big of a deal! But as I've always done with any trip we take, I plan ahead and plan smart. Go with flexibility and common sense.
First of all, taking a child under 3 is great because they are FREE! Who doesn't like free??? If you do the dining plan, children can eat from your plate or if it is a buffet, they eat off the buffet for free. We always brought food for the kids- things like fruit cups, pb crackers, dried fruit, trail mix, breakfast bars, etc.. Things that had nutritional value and they could eat as meals. That way we weren't limited on what we could order. This upcoming trip will be our first paying for our son, which stinks on some levels. (Yuck...over $200 for his park pass!) But it will be nice to not have to bring tons of food with us and he gets all his own counter meals like his sister.
WDW is THE MOST kid-friendly place on the planet. We've taken other vacations with the kids and it's not as easy. In Disney, there are kid care centers in each park. A place for parents to feed, change, play with the little ones if they need a break and some quiet time. A large percentage of attractions in the parks are suitable for kids of all ages. Try going to a Six Flags or other amusement park...there's very little for parents to do with small children. And most of it the parents cannot also ride with the children. The cast members who work at WDW know how to deal with children. They aren't uncomfortable around little ones. They know how to speak to them to make them smile.
I know many people use the argument of "the kids will never remember." Yeah, probably not. But the kids sure have a blast while they're there. The parents have fun and have memories of the fun had. Family time is very important for our family, and vacations are too. It's very hard to completely relax and forget about bills and responsibilities while at home. So for us, leaving home is a big part of complete rest and relaxation. We enjoy doing fun things with our kids and so what if they don't remember. We do. And I have the photos to prove it.
I know for some families, a Disney trip is a once in a lifetime event, so yes, it is smarter to wait until the children are old enough to fully take advantage of everything the parks have to offer and be old enough to remember it. If that is the case, I think 5-7 is a great age. They are still young enough to "get" the magic and still believe, but should be tall enough to ride most rides. They'd still have to skip some though. Our daughter has always been a peanut, so at 9, she is finally tall enough for Rock n Roller Coaster (minimum height requirement is 48"). And if we'd waited til she was this age to take her for the first time, we would have missed out on all the memories she's made, playing with the characters, still believing that she was meeting the REAL Mickey and Cinderella. She's past that now...she knows there's someone inside the costume. She knows they're actors. I wouldn't have wanted to give up the memories of her smiles and the excitement in her eyes meeting them when she was 2-1/2 and 4.