My son has been wanting to participate in JTA since he was two.
He actually did it last year about two months after he had turned four, and he loved it. In fact, he loved it so much that we changed our plans and gave up a second morning at MK to go back and do it a second time. The first time he got to "fight" Darth Vader, and the second time, they had a surprise villain come out, Darth Maul, and he got to "fight" him too.
He absolutely loved it, and it was all he talked about for months. He still talks about it.
We are going again to WDW again this September, and he is actually preparing for it! So I think it just depends on the child, as far as age goes.