Stonfiesta,
I know you didn't ask me, but I'll answer anyway!
Personally I wouldn't take a child to WDW until they are old enough to appreciate it.
We took DS1 when he was 4 to WDW. He was brilliant on the two flights, but was totally over awed by the whole experience of the MK, Epcot and the Studios. (We didn't even do Universal or the water parks- we just stuck to the 3 I mentioned). Every night he had massive melt downs because he was too over loaded, (despite the fact we only spent about 3- 4 hours a day at the parks, had plenty of rest days, naps etc). He loved meeting the princesses, Tinkerbelle etc and loved the rides, but it totally blew his mind and ended up ruining our holiday TBH. He had a massive temper tantrum in one of the shops and couldn't deal with the whole experience, despite us preparing him well, and taking it at his pace.
In the end we hardly saw anything and I now wish we had waited until he was this age- almost 6. It's alot of money to spend if it doesn't work out. We had saved for 5 years for this trip and I am still disappointed with how it all turned out and wish I'd waited, as it's a lot of money to blow on a holiday.
We do have some fab photos of Ds with the characters, and he got to drive the monorail, steer the Jungle Cruise etc, and I do have some brilliant memories, so it wasn't a total waste, but it could have been sooo much better if I'd been patient and waited 2 years more.
Also, at 3 he wasn't 100% potty trained, so I wouldn't have risked the flight.
I guess it just depends on each individual child.
I should just add, we've been taking him to DLP since he was 14 months old, and he loved every minute of it.