I know people take babies seemingly straight out of the womb to WDW, but I would wait until the baby is big enough to sit up in a stroller. And be careful of the heat. We started taking our DSs when the youngest was 11 months and he did great! He was big enough to enjoy what he was seeing and not so small that he was simply a hindrance to our touring and riding. He gazed at Piglet like they were soulmates and enjoyed all the attractions. He was happy to sleep in the stroller whenever he needed to, so we just kept going. I spent many rounds on the PeopleMover while he slept too! For us, 11 months was perfect (but we could have easily taken him a couple of months earlier too)!
Do you ride the MK railroad and if so, do you ride it as transportation or as an attraction itself?