I think it just varies from campus to campus and how many spots the chapter has left to fill. I was a transfer student to EKU when I went through recruitment, I was a 22 year old Junior who was behind (I went alum last May and graduated this past December(because I didnt need a full load to graduate so I was a part time student last fall). Anyways, I was 22 and I got in. However I know I most likely wouldnt have at my previous school. It just depends on the school and the chapter. There are several threads about this in the Rush forum. They may be worth checking out
Good luck with whatever you decide to do