I did my first program in Spring 2012, during the second semester of my junior year, and then did my second program in Fall 2013, after I graduated college (May 2013).
I did it later in college, and was lucky to have been accepted on both my first and second tries. I can, however, see advantages to applying earlier in your college career. One is that if you don't get in on your first try, you have more chances to apply in the following semesters. Also, if you decide you love what you are doing and are able to stay on in your role/elsewhere in the resort, it's usually easier to transfer schools in your first year or two.
However, I'm personally glad that I was a bit older and more mature during my programs. I had lived away from home even before college (at sleepaway camp every summer since I was 9), and then lived away at school with roommates before the CP, but it's hard living away from home for the first time, and in Disney you're most likely going to have a lot of roommates, so it's nice to have that experience before arriving and dealing with the added stress of a job that is likely high pressure!
Hope that helps with your decision.

Good luck!