We just did Jedi training this week and it was great! My 7 yo DS loved it.
Just to reiterate what everyone said...get there early! The family in line ahead of us were trying for a second time. The first time they tried they got there at 9:10 and it was already too late. I think we got to the park about 20 minutes before park opening on an easywdw "recommended" day. I took my DS to sign up while DH went to grab FP- for TSM.
The minimum age is 4 yo. I really don't recommend trying to fudge the age, even if your kid is really close to 4. In fact, I wouldn't recommend it for a young 4 yo unless they are really brave and good at following directions. The youngest kids on the stage all had a hard time and we saw some that were too scared to complete the training once Darth Vader/Maul comes out. Considering only around 15 kids get to do each show, it's a real shame to see a slot taken by a kid who isn't really mature enough.
The CMs were asking the kids (not parents) their ages and for the very young ones, asking them if they could follow directions.
The line took a bit of time, but you won't have any trouble getting out to go to the bathroom. I think it would be best if you had two adults just so one of you could stay in the line while the other goes to the bathroom with the child.
Oh, and don't walk right pass the sign-ups, like I did. I was so focused on the stage that I didn't notice the building with the sign-ups. It's on your right-hand side--look for it as soon as you can see the stage where the show happens. There will be CMs with signs and of course, a small crowd of people. In that part of the park, I don't think that there is anything else that generates a crowd that early in the morning.