Go now. They are the perfect age for Disney World. My daughter, age 8 1/2, and my son, age 6, have been 4 times already. Their first trip was when my daughter was 3 and my son was 4 months old. And we drove down there from Michigan. My daughter loved it. We did have grandma and grandpa with us while we were at Disney which made it a little easier.
We also went when my daughter was almost 6 and my son was 3. Again when my daughter was 7 and my son was 4 1/2. And again when my daughter was 7 1/2 and my son was 5. We flew down for these trips. And we are looking to go again sometime in the next year.
As others have said, as the kids get older, the magic starts to go away or at least change. My daughter still wants to go to Disney, but she's pretty much done with princesses. She already told me that she doesn't need to do the princess lunch anymore unless it's in the castle. In all of our previous trips, she was in awe of the princesses and couldn't wait to see them.
As far as the rides go, we never had a problem with the kids not being able to go on a ride that they wanted to go on. There are so many things to do that even if there are rides that they aren't big enough to ride, there are more than enough things to do to fill your days.