My DD is 7 (almost 8). She has been to WDW 9 times, spread pretty evenly over her lifetime. Her first visit was at 5-1/2 months. She had a ball - she loved the lights, sounds, people, activity, characters, etc. Of course, she doesn't remember a moment of it, but mom and I certainly have wonderful memories of that trip.
Her next visit was at 1 year. This was neat because she was walking and could explore a lot more which she loved doing. She loved the characters and all the rides.
At 2, she really started to get into things more. By 3 and 4, memories started sticking and she still recalls things from those visits.
Before we had a child, we swore we wouldn't take one there until they were 5 or so. But once we had our DD, we weren't willing to not go because we had her. Every trip with her has been terrific in its own way. As she has grown and matured, we've gotten to see her experience the trip in a new way and on a different level. On our last trip, she got to ride the coasters for the first time and she was so excited and proud of herself. Also, as she is getting older, we're able to expose her to some of the nicer restaurants which is good for DW and I because neither of us particularly likes fast food.
I guess that doesn't really answer your question, but I really think that every age is good as long as you go at the child's pace and don't force him/her to go at your pace.
Steve