We took DS for the first time at 7 1/2 months. We just did one day at MK, went with low expectations on what we would do, and we ended up having an incredible time (rode a ton of rides with him). He got all excited getting his first Mickey ears (see my photo), and he was familiar with Mickey. Well, when we went to meet him, he was so funny. He recognized him and immediately started kicking his feet and waving his arms in excitement.
It was also very easy to spend time in the park with him. He was in the stroller most of the time, and a front-facing carrier really helped when in lines and on rides. It was easy to get on and off rides that way, and he was able to see everything facing forward. A backpack also worked better than our regular diaper bag. Also, when he napped, DH and I would do rider swap on the rides with height restrictions.