I don't have kids yet, but that always seems like such a weird question to me.
I mean, by that logic, why do we do *anything* special with young children?
We do hope to have the budget to take our babies to Disney before they're 2. I mean, why not? They're free all around. It'll make great pictures, and it'll be memories for us. I think like most people have said before, it's more for you than it is for them - and that's 100% okay.
Magic only lasts so long in the life of a child, at some point they realize that maybe there isn't just one friend of Mickey, and all that glittery stuff in their hair isn't pixie dust (I mean, Tink really doesn't have that much to spare now does she?). So might as well give them as much of it as possible while they still wholeheartedly believe in it, even if it'll be foggy in their mind as they get older. Besides, by those standard, what is the right age to take a child on vacation? My cousin went when she was 8, and she doesn't remember that much of it either.
We're taking our little cousin this fall, and she'll have just turned 3. I highly doubt she'll remember any of it, but I'm quite certain she'll be very happy to be there while we're there, to meet Merida and run around Magic Kingdom, and we will have the pictures to remember it by when she's older.