How young is too young to take children to Disney World? There is no too young. I've taken babies from 4 months old on up and even they enjoy it.
What age is the "right" age? There is no right age. All ages are right. Every age for every child is different and a totally new adventure. At 4 months, Disney World is big and bright in the eyes of the baby. It is something to stare at and listen to with all the amazing sounds. At 9 months old, Disney World looks like a great place to play. At 2 years old, some character (or characters) has emerged as a definite favorite, if not before making Disney World a place to find your favorite character and a magical place to be. All of the preschool years are a time to learn about what is there and growing just big enough to ride the next great attraction. Early school years are for seeing how brave they can be. Can they do Space Mountain or will they have to settle for Goofy's Barnstormer?
Pre-teen years are for trying out boundaries and seeing what they can get away with. Well, that's at home. At Disney World, it's totally different. You're doing something for them and with them and there is no place they would rather be than exploring it with you. Teen years, that would normally be spent locked in their room, is a time for you to have fun doing things together. Exploring new things and having a blast forgetting all the issues you face at home--like boyfriends and homework!
Each age is a new adventure and a very special time to go. I've been lucky enough to take my children and grandchildren many times (the 18 month old has been three times already!) and every time is a new adventure for us all. With all the trips we've had, there is always something new to do.
Whatever age you are or those going with you, take it as a new adventure. Don't compare it to your last trip or to trips you may have in the future. The magic will always be there for you--just open your eyes and see it in the children's eyes.