IMO people who say kids are too young have never been to Disney, or if they have they are just too uptight! My daughter is 4.5 and has been 6 times, my son has been 4 times and he'll be 3 in April. We live in Seattle so it's not like it's an easy journey, but once we are there I can't think of a better place to be!
My daughter was 7 months her first trip and that was great. My son 5 months, that was a bit harder because he couldn't sit up so it was more work to have to hold him on every ride (he was a big kid), but we wouldn't change it for anything.
The key with small kids is to realize you aren't going to see everything, and to recognize you kids limits. We always go back and take naps, or at least take a long break in the afternoon, maybe have ice cream, watch a show etc. Also don't expect to go on the big kid rides. I don't feel it's fair to make them wait so I can ride space mtn or whatever. We do those if the kids fall asleep in the stroller or the grandparents can take them on other rides.
We have gone in Spetember for the last 3 years. It's VERY hot, but there are NO lines. The first September we rode Buzz, Teacups (3 times) and Pooh in about 30 minutes. If there are no (or minimal lines) I will let my kids ride the same ride as many times as they want. My daughter starts Kindergarten this year so we will have to pick a new time to go. We are so sad that we can no longer go in Sept!