Small Children
Is X age too young to go to Disney?
In a word, no. While I don't know that I would plan a trip specifically to take a very small (read: pre-walking) child to Disney, I wouldn't hesitate to bring a small child on a trip, and there are wonderful experiences for all ages.
The biggest objection is typically "I don't want to take them if they aren't going to remember the trip," or "I want to wait until they are tall enough to ride everything," and those honestly fair concerns. With that said, there are few things like experiencing Disney through the eyes of a child that legitimately thinks they are meeting Cinderella or Mickey Mouse, and you will have those memories. So, unless you truly expect it to be a "once in a lifetime" trip, feel free to take children of any age. Though, be aware that sometimes, those "once in a lifetime" trips can lead to "well, we've got to come back here regularly." :)
And if you ARE going on a trip with very small children, just know that you'll enjoy the trip a lot more if you slow down a bit and let the trip come to you. Keeping the same pace you might keep with teenagers is unrealistic, and small kids in particular could very well be having just as much fun playing the drums or on a small playground as they would on a multi-million dollar headliner attraction.
Finally, consider bringing or renting a stroller even if your kid doesn't continue to use one at home. The amount of sustained walking you do at Disney dwarfs what most kids are used to at home, and it's not uncommon to see kids who are 6 or 7 years old being pushed around in strollers.