We took our grandson and his mon and dad when he was just 18 mo, he will turn five this month. He really seemed to enjoy all of the activity, colours, characters and things to do. Since that first trip, he has gone every year since. Each year he has gotten more into Disney just like his mom and dad and grandparents do. I trully do beleive that at any age they will have fun. Take lots of pictures of them with the characters, and doing things in the parks. Later on when they look at the pictures you can tell them about what was happening and all the fun the family had then. Then the next time you go, pull those pictures out again and talk about this ride, or character or event and use it to help them get excited about going again and doing all those neat things again and adding some new ones.
Don't forget that not only are you building memories for your kids, but for yourself and family. That's why I say it's never to early to take a kid to Disney. They may not remember the trip but only through the photos you've taken and what you tell them about it, but YOU will remember every moment and how special it was for YOU with them!
Special note...On the grandson's second trip to Disney, we stayed at the WL. I'll never forget the look on his face when he walked into the lobby the first time. I was just ahead of he and his daddy, they were holding hands just walking into the lobby. Whitt (grandson) looked up and all around, stood there in amazement and then turned to his daddy and said "WOW!". For me that is a moment I'll never forget. That's what Disney is about, building family memories!!!!