I suppose you need to find the magic before you can lose it. Remember that most kids never go to WDW (or DLR) at all. I know, I was one of 'em.
But I'm thinking that it all depends on what you call the magic. If it's believing that the characters are real, well, even as a little kid when DLR was new, I knew that they were people in costumes because all the articles about this new park went into great detail. I suspect that for most kids, this probably happens around 6 or so, when they start questioning Santa Claus and the Easter Bunny. (They're both real, by the way, and don't try to convince this old soldier any different.
)
If it's believing that the parks are special places, though, where ordinary life isn't allowed in -- I believe that now MUCH more than I ever did as a kid. That's why I'm here in the first place. And I think that's where the REAL magic is, and I don't know that there's any special age where your kids might stop believing in that. If there is, then I feel sorry for 'em because they're all grown up then, and growing up is nothing but a desperate effort to deny your dreams and imagination.
And for the same reason, our (nearly) 22 year old son has been asking when we're going ever since I told him that it had come of the list of "not in my lifetime" items. He's more fired up than I am.
So, let's see... It may end at some point past 60, but I'm not there yet.