We took our oldest DD to WDW for the 1st time (not counting while I was pregnant) when she was a little over 8 months old. Great time if you ask me, they can sit up and enjoy things, but are really easy still. (if you follow their lead.)
Our 2nd child went at 7 months old, and again at 9months old. Both great trips. I think she enjoyed 9 months old trip a bit more, but we all had a blast.
I think the time of year you go is more important then the childs age. I like the winter months when it isn't as hot, rainy or crowded. Then again May is also a great time to go.
Being prepared is good. It was very easy for us because we know WDW well and know the ins and outs and tricks of the trade.
It was a lot easier for us when she was 7 and 9 months old then the most recent trip she was 17 months old. That was a fun trip, but more work for us. (she loved it.) We might go back in May, she will be 20 months old and her big sister 5 years old. It will be her big sisters 8th trip to WDW (I think, we may have lost count.
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Personaly I think over 12 weeks is good, but never say never. If I had an opprotunity to go when a baby was younger and not later, I might go for it (if all the other situations were right.) If it is your 1st baby I think the real requriement is that you both are ready for travel, comfortable with things and each other, all healed from the delevery and baby and parents are easy going about things. Breast feeding is much easier too! (not carrying food with you, you have all you need with you were ever the baby is hungry.
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It does apear that babies that go to WDW early might not remember the trip, but they grow up to be WDW fanatics!