What is a good age to take the kids to disney world if you know you can only go once?

I would probably take them right around the 9 years age mark. At 9 they can enjoy most of the rides and for most kids are still young enough to enjoy the magical part of it all. If you have kids that range in different ages I would try to get it to where maybe the middle child is right about that age (unless the childrens age differences are many years in between) that way they can all hopefully enjoy it! Were planning on taking our 2 kids when they are 7 and 9 to Disney Land that way they can enjoy hopefully all aspects of it and if we can never go back at least they'll remember it!