I have taken kids from 8 months to age 10. Every age has been fun. However, the most challenging parenting times for us have been with our 3.5 year old, and just turned 4 year old. Developmentally, this age is beginning to have a good grasp on time, typically has routines at home that are harder to break from, and has the desire to keep up and go, go, go, but not the necessary stamina. They also are at an age where they can remember things and ask for them and not understand why they can't have them right now. (Read doing a ride again when you are in another park). You can have a great trip, but be prepared for needing to do less than you would with an older child. Even with a child just turned 3, we had less difficulty. This could just be my children's pattern, but it was interesting that they both entered the phase of what I would term "challenging travelers" at the same age. (The trips in question were 4 years apart.). I found babies and younger toddlers much more go with the flow. On the other end, by 6 my older kids have been more patient, flexible, and fun travelers.