Sure a 15 mo old won't remember going to WDW. So, if the main criteria for taking a toddler to the parks is simply will they remember it, that could color that decision. I don't think that's a necessary standard, since parents are going to do something with their young children and going to WDW seems like as good an option as anything else one would want to do together as a family. I don't see the downside to taking a toddler to WDW where they can ride a lot of rides, meet characters and watch age appropriate parades and shows -- as long as you are fine with the "hassles" that come with bringing a child to the park.*
Anyway, if you are using "remembering the trip" as a criteria for having a child go, you are still looking at something like 2 1/2 for an age cutoff where the will have recollection of stuff. At worse, by around 3 years old, a child will remember a lot. I think it's hard to come up with a vacation/family time that is more ideal for a 3-5 year old than Disney and a child at that age is certainly going to require a stoller (to bring it back to the initial issue in this thread) given the distances involved in walking at Disney.
*=I would submit that non-parents probably overestimate how much the work involved in having a young child is at the parks. Simply put, the feeding the kids, changing them, watching them all the time, carrying them, whatever -- it's stuff you do every day. If you don't have children and you see folks do it at a park, it might seem like it is something that is a big burden and interfering with a vacation, but the reality is that it becomes second nature for a parent of a young child to do that stuff. And if you are going to be changing a poopy diaper anyway, much rather do it at a magical place like WDW where it is fun for the parent as well as the kid.