We did this a couple of months ago with our 4-year old; we're locals, and literally came to the park to do this and head home. We arrived at the park at opening with no EMH that day, and headed straight to the sign-up - the first time they could offer us was 3pm! So definitely get there early, especially if you want an early time, and prepare to be flexible with times.
A CM told us that we could give up our guaranteed 3pm slot and wait around at the earlier times to see if anyone didn't show up, so that's what we decided to do. We were incredibly lucky that someone didn't show up for the very first show, so we were able to get in there at the last minute. They were very strict on timing; one kid needed to go to the bathroom and the CM told him "you've literally got less than 2 minutes or we'll leave without you!" His dad picked him up and sprinted to the bathrooms with him!
4-years old is the minimum age, and the two 4-year olds in the group (my son and another little girl) were definitely a bit intimidated. The CMs were amazing though; my son saw Darth Vader and looked terrified, but one of the CMs was right there talking to him and telling him not to worry. He ended up battling the Seventh Sister instead, and although I don't think he'll be winning any light saber battles int he future, he really enjoyed it.