They are free until age 3, so we took DD at 2 years 10 months. She is 14 now and we have been to WDW or DLR (or both) every year since. That 2 year old trip is still one of my absolute favorite trips, if not my favorite trip. Everything was new, real, and exciting to her and she absolutely loved all the characters. She was what they call a "spirited" child (LOL) and she was much better behaved at WDW than she was at home. It was so fun to watch everything through her eyes. I remember walking into MK the first day expecting her to be in awe of the castle- didn't happen. She could have cared less about the castle. She walked in and spotted Pluto. That wild child waited patiently in line for 20 minutes to see Pluto with no complaints and then threw herself at him and hugged him like he was her long lost best friend. One of my favorite Disney moments ever. I had visions of taking her on Dumbo for the first time. She took one look and pointed up and said "I not riding him". We didn't now until later that it turned out that she was afraid of heights. LOL. Aside from heights, she was a lot more fearless about rides at age 2 and 3, than age 5 and 6. When they are little you have to be flexible and be prepared to modify plans based on what works for them and what they like, but it can be absolutely magical.
Either age 6 or 7 was the worst age for us. She was too old/big for a stroller, but didn't quite have the stamina for all the walking and got hot, tired, cranky and whiny pretty quickly. By age 8, her stamina was built up and all was good again. I would much rather take a 2 or 3 year old than a 6 or 7 year old based on our experience. LOL. In kindergarten their friends with older siblings start enlightening them on what is real and what is not-- so it was great having a few trips while everything was real and magical.