My dd was 2 1/2 when we brought her and my little one was 2. I have never regretted my decision to take them that young. They may not have been able to or still can't go on some attractions. Theyare 6 and 8 and still don;'t meet some heigt requirements, but we have always had great times.
For us, I wantesd to experience the magic of them seeing princess's for the first time and Mickey Mouse, while they still "beleive". I will always have the picture of my daughter's expression from seeing Ariel for the first time and my youngest could barely talk when she was 2, but sang to Sleeping Beauty at the Storybook Princess Breakfast. They both don't remember their first couple of times, but i do. I will always have those memories. There is nothing like that innocence. I can't imagine going for the first time when they are oldter because i wouldn't want to miss that excitement for the world. I went when i was 6 and every year after and i know that the feeling changes. I have always loved Disney and everything about the experience, but its different at different ages
Yes i know you can still beleive and have that Disney magic when you are older, but it's not the same. That is what i wanted for my dd's. I know there are plenty that don't feel the same. It all depends what you want for your children.