The age old question of when it's a good time to take them for the first time......it will differ entirely on who you ask.
I have to say, I've taken mine as young as 6 mo. for their first trip up to 16 months old is the latest I've taken them for a first time. For me, my thought process has always been a bit different about this. Instead of looking at it as "I want to wait until they remember", I want my kids to always have remembered Disney (whether it's DLR or WDW) as a part of their lives. When they're looking back on their lives, I don't want there to ever be a time when they don't remember Disney as being part of it. And, I have to say, both my DH (who as you'll see from my siggy isn't as....well, enthusiastic about Disney as I am) look back at our favorite Disney trip and we both think about our oldest son's first trip at 16 months. (And, my first trip to Disney was when I was 1 and I have a ton of memories of lots and lots of trips.) My DH and I were both in law school, very busy, DS had already had two of the four surgeries that he ended up needing before he was 4, I had just found out I was pregnant with my DS2...we needed some family time that we just focused on us. I worked for the
Disney Store so was able to get discounts on lodging and food and merchandise, DS was free to travel, I had a frequent flyer ticket. The trip was so magical. DS loved the characters. Because we had him, I think we took better care of us. No commando style, we went to the parks in the morning and stayed until early afternoon. Then we went back to the hotel and napped, played in the pool a bit, walked around, ate dinner at the hotel and then returned to the parks to watch parades or fireworks, etc.
That being said, I think the magic window for children is somewhere between the ages of 2-7. Two being perhaps a bit young for some children, and 7 being a bit too old. I think that's the age range where everything is still magical, the characters are 'real', little things like the lights make them smile, the rides in Fantasyland are the big lure, and the hippos are scary on Jungle Cruise. Well, you get the idea. If we were thinking Disney would be a once in a lifetime, or even once in a decade kind of thing, then I'd shoot for somewhere in the 5-6 range, myself.