my son went as a fetus, then when he was 6 weeks old, then when he was 6 months old, then when he was a year old, 18 months, 2 years, 2.5 years, 3 years -- and then alas, we moved overseas, when the visits became yearly...
my daughter also began her visits as a fetus etc..(same as above)..
clearly there is a big difference depending on the age....but from about 2 years old it started to make a huge impression on them.....their first actual memories that they retain to this day (they're 23 and 20) are probably from about 3 years....my daughter still remembers the melvin the moose breakfast and she was 3 years old at that....
as for how it was, in terms of manageability, it was always really easy....at every age....we always took it easy.....did whatever the kids wanted to do....and we rested a lot...we only went to the parks in the morning....and then again in the late afternoon, after relaxing/swimming (my kids never napped, so they didn't nap at WDW either)...
as i recall, my kids always liked to do things very slowly when they were young.....and they sort of liked to stop and smell the roses so to speak (look at all the little theming sort of things)......so it was never commando style when they were young....